fix(task): make task dev / task dev:all work on Windows (#6392)

## Summary

`#6145` (port picker) and `#6244` (gradle unification) combined to break
`task dev` / `task dev:all` on Windows. Two independent regressions,
both addressed here.

### 1. `find-free-port.ps1` panic (`#6145`)

```
$ task --dry dev
The argument 'scriptsfind-free-port.ps1' to the -File parameter does not exist.
panic: ended up with a non-nil exitStatus.err but a zero exitStatus.code
```

- **Backslash stripped.** `Taskfile.yml` had
`scripts\find-free-port.ps1`. go-task pipes the `sh:` block through
mvdan/sh, which treats `\` as a POSIX escape and silently drops it,
leaving `scriptsfind-free-port.ps1`. PowerShell can't find the file,
exits non-zero, mvdan/sh panics on the inconsistent exit status.
Switched to a forward slash.
- **`-Preferred 8080,5173` is brittle.** Relies on PowerShell parsing
the comma-list into `[int[]]`. Dropped the named flag; switched the
script's `param` to `[Parameter(ValueFromRemainingArguments =
$true)][int[]]$Preferred`; pass each port as its own positional token
(matching how the bash variant is called).

### 2. `bash gradlew` can't find Java on Windows (`#6244`)

```
[backend:dev] ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
```

`#6244` unified all gradle invocations under `bash gradlew` on the
assumption that Git-Bash inherits Windows-side env. It doesn't (and
neither does WSL bash, which can also shadow `bash` on a developer's
PATH). The standard Adoptium/Temurin installer sets `JAVA_HOME` in the
Windows env only, so `bash gradlew` fails before Spring Boot even loads.

Restored the per-platform branch for every backend task: `cmd /c
".\gradlew.bat ..."` on Windows, `./gradlew ...` on Linux/macOS. Two
Windows-specific gotchas to be aware of for future edits:

- mvdan/sh strips `\` outside of double quotes, so `cmd /c
.\gradlew.bat` ends up as `.gradlew.bat`. The entire payload must be
wrapped in double quotes (`cmd /c "..."`).
- Modern Windows excludes cwd from cmd.exe's search path, so the leading
`.\` is required — bare `cmd /c gradlew.bat` errors with "is not
recognized" even from the repo root.

## Test plan

- [x] `task --dry dev` / `task --dry dev:all` on Windows resolve ports
cleanly and dispatch the inner tasks
- [x] `powershell -NoProfile -File scripts/find-free-port.ps1 8080 5173
5001` prints three ports
- [x] `task backend:dev PORT=8081` on Windows starts Spring Boot in ~9s;
`/api/v1/info/status` returns `{"status":"UP"}`
- [x] `task dev:all` on Windows brings up all three services healthy:
backend `/api/v1/info/status` 200, engine `/health` 200, frontend `/`
200
- [ ] Linux / macOS path unaffected (only the Windows branches changed
in behaviour)

.taskfiles/backend.yml

@@ -1,7 +1,19 @@
 version: '3'
 
-# All gradle invocations go through `bash gradlew` (the sh wrapper) so they work
-# uniformly on Linux/macOS (native bash) and Windows-with-Git-Bash.
+# Gradle invocation strategy:
+#   - Linux/macOS: `./gradlew` runs the POSIX shell wrapper natively.
+#   - Windows:     `cmd /c ".\gradlew.bat ..."` invokes the .bat wrapper through
+#                  cmd.exe so it inherits the user's Windows-side `JAVA_HOME`
+#                  and PATH. Routing through `bash gradlew` on Windows ends up
+#                  under WSL or Git-Bash, neither of which inherits
+#                  Adoptium/Temurin's default Windows-only Java env - gradlew
+#                  then errors with "JAVA_HOME is not set and no 'java' command
+#                  could be found".
+#
+#                  The entire `.\gradlew.bat ...` payload is double-quoted so
+#                  the leading `.\` survives mvdan/sh's POSIX backslash
+#                  stripping; cmd.exe also requires `.\` (not bare `gradlew.bat`)
+#                  because modern Windows excludes cwd from cmd's search path.
 
 tasks:
   dev:
@@ -13,43 +25,67 @@ tasks:
     env:
       SERVER_PORT: '{{.PORT}}'
     cmds:
-      - '{{if .AIENGINE_URL}}AIENGINE_URL={{.AIENGINE_URL}} AIENGINE_ENABLED=true {{end}}bash gradlew :stirling-pdf:bootRun'
+      - cmd: '{{if .AIENGINE_URL}}AIENGINE_URL={{.AIENGINE_URL}} AIENGINE_ENABLED=true {{end}}cmd /c ".\gradlew.bat :stirling-pdf:bootRun"'
+        platforms: [windows]
+      - cmd: '{{if .AIENGINE_URL}}AIENGINE_URL={{.AIENGINE_URL}} AIENGINE_ENABLED=true {{end}}./gradlew :stirling-pdf:bootRun'
+        platforms: [linux, darwin]
 
   dev:bundled:
     desc: "Clean + bootRun with frontend bundled into the backend (single :8080 server)"
     ignore_error: true
     cmds:
-      - bash gradlew clean bootRun -PbuildWithFrontend=true
+      - cmd: cmd /c ".\gradlew.bat clean bootRun -PbuildWithFrontend=true"
+        platforms: [windows]
+      - cmd: ./gradlew clean bootRun -PbuildWithFrontend=true
+        platforms: [linux, darwin]
 
   build:
     desc: "Full backend build"
     cmds:
-      - bash gradlew clean build
+      - cmd: cmd /c ".\gradlew.bat clean build"
+        platforms: [windows]
+      - cmd: ./gradlew clean build
+        platforms: [linux, darwin]
 
   build:fast:
     desc: "Build without tests"
     cmds:
-      - bash gradlew clean build -x test
+      - cmd: cmd /c ".\gradlew.bat clean build -x test"
+        platforms: [windows]
+      - cmd: ./gradlew clean build -x test
+        platforms: [linux, darwin]
 
   build:ci:
     desc: "Build for CI (formatting checked separately)"
     cmds:
-      - bash gradlew build -PnoSpotless
+      - cmd: cmd /c ".\gradlew.bat build -PnoSpotless"
+        platforms: [windows]
+      - cmd: ./gradlew build -PnoSpotless
+        platforms: [linux, darwin]
 
   test:
     desc: "Run backend tests"
     cmds:
-      - bash gradlew test
+      - cmd: cmd /c ".\gradlew.bat test"
+        platforms: [windows]
+      - cmd: ./gradlew test
+        platforms: [linux, darwin]
 
   format:
     desc: "Auto-fix code formatting"
     cmds:
-      - bash gradlew spotlessApply
+      - cmd: cmd /c ".\gradlew.bat spotlessApply"
+        platforms: [windows]
+      - cmd: ./gradlew spotlessApply
+        platforms: [linux, darwin]
 
   format:check:
     desc: "Check code formatting"
     cmds:
-      - bash gradlew spotlessCheck
+      - cmd: cmd /c ".\gradlew.bat spotlessCheck"
+        platforms: [windows]
+      - cmd: ./gradlew spotlessCheck
+        platforms: [linux, darwin]
 
   fix:
     desc: "Auto-fix backend"
@@ -59,7 +95,10 @@ tasks:
   swagger:
     desc: "Generate OpenAPI docs"
     cmds:
-      - bash gradlew :stirling-pdf:copySwaggerDoc
+      - cmd: cmd /c ".\gradlew.bat :stirling-pdf:copySwaggerDoc"
+        platforms: [windows]
+      - cmd: ./gradlew :stirling-pdf:copySwaggerDoc
+        platforms: [linux, darwin]
     sources:
       - app/core/src/main/java/**/*.java
       - app/proprietary/src/main/java/**/*.java
@@ -77,19 +116,31 @@ tasks:
     desc: "Print project version"
     silent: true
     cmds:
-      - bash gradlew printVersion --quiet | tail -1
+      - cmd: cmd /c ".\gradlew.bat printVersion --quiet" | tail -1
+        platforms: [windows]
+      - cmd: ./gradlew printVersion --quiet | tail -1
+        platforms: [linux, darwin]
 
   licenses:check:
     desc: "Check dependency licenses"
     cmds:
-      - bash gradlew checkLicense --no-parallel
+      - cmd: cmd /c ".\gradlew.bat checkLicense --no-parallel"
+        platforms: [windows]
+      - cmd: ./gradlew checkLicense --no-parallel
+        platforms: [linux, darwin]
 
   licenses:generate:
     desc: "Check and generate dependency license report"
     cmds:
-      - bash gradlew checkLicense generateLicenseReport --no-parallel
+      - cmd: cmd /c ".\gradlew.bat checkLicense generateLicenseReport --no-parallel"
+        platforms: [windows]
+      - cmd: ./gradlew checkLicense generateLicenseReport --no-parallel
+        platforms: [linux, darwin]
 
   clean:
     desc: "Clean build artifacts"
     cmds:
-      - bash gradlew clean
+      - cmd: cmd /c ".\gradlew.bat clean"
+        platforms: [windows]
+      - cmd: ./gradlew clean
+        platforms: [linux, darwin]