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)

Taskfile.yml

@@ -3,7 +3,7 @@ version: '3'
 output: prefixed
 
 vars:
-  FIND_FREE_PORT_PS: powershell -NoProfile -File scripts\find-free-port.ps1 -Preferred
+  FIND_FREE_PORT_PS: powershell -NoProfile -File scripts/find-free-port.ps1
   FIND_FREE_PORT_SH: bash scripts/find-free-port.sh
 
 includes:
@@ -45,7 +45,7 @@ tasks:
     desc: "Start backend + frontend concurrently on free ports"
     vars:
       PORTS:
-        sh: '{{if eq OS "windows"}}{{.FIND_FREE_PORT_PS}} 8080,5173{{else}}{{.FIND_FREE_PORT_SH}} 8080 5173{{end}}'
+        sh: '{{if eq OS "windows"}}{{.FIND_FREE_PORT_PS}} 8080 5173{{else}}{{.FIND_FREE_PORT_SH}} 8080 5173{{end}}'
       BACKEND_PORT: '{{index (splitList "\n" .PORTS) 0}}'
       FRONTEND_PORT: '{{index (splitList "\n" .PORTS) 1}}'
     deps:
@@ -62,7 +62,7 @@ tasks:
     desc: "Start backend + frontend + engine concurrently on free ports"
     vars:
       PORTS:
-        sh: '{{if eq OS "windows"}}{{.FIND_FREE_PORT_PS}} 8080,5173,5001{{else}}{{.FIND_FREE_PORT_SH}} 8080 5173 5001{{end}}'
+        sh: '{{if eq OS "windows"}}{{.FIND_FREE_PORT_PS}} 8080 5173 5001{{else}}{{.FIND_FREE_PORT_SH}} 8080 5173 5001{{end}}'
       BACKEND_PORT: '{{index (splitList "\n" .PORTS) 0}}'
       FRONTEND_PORT: '{{index (splitList "\n" .PORTS) 1}}'
       ENGINE_PORT: '{{index (splitList "\n" .PORTS) 2}}'