Add Taskfile for unified dev workflow across all components (#6080)

## Add Taskfile for unified dev workflow

### Summary
- Introduces [Taskfile](https://taskfile.dev/) as the single CLI entry
point for all development workflows across backend, frontend, engine,
Docker, and desktop
- ~80 tasks organized into 6 namespaces: `backend:`, `frontend:`,
`engine:`, `docker:`, `desktop:`, plus root-level composites
- All CI workflows migrated to use Task
- Deletes `engine/Makefile` and `scripts/build-tauri-jlink.{sh,bat}` —
replaced by Task equivalents
- Removes redundant npm scripts (`dev`, `build`, `prep`, `lint`, `test`,
`typecheck:all`) from `package.json`
- Smart dependency caching: `sources`/`status`/`generates`
fingerprinting, CI-aware `npm ci` vs `npm install`, `run: once` for
parallel dep deduplication

### What this does NOT do
- Does not replace Gradle, npm, or Docker — Taskfile is a thin
orchestration wrapper
- Does not change application code or behavior

### Install
```
npm install -g @go-task/cli    # or: brew install go-task, winget install Task.Task
```

### Quick start
```
task --list       # discover all tasks
task install      # install all deps
task dev          # start backend + frontend
task dev:all      # also start AI engine
task test         # run all tests
task check        # quick quality gate (local dev)
task check:all    # full CI quality gate
```

### Test plan
- [ ] Install `task` CLI and run `task --list` — verify all tasks
display
- [ ] Run `task install` — verify frontend + engine deps install
- [ ] Run `task dev` — verify backend + frontend start, Ctrl+C exits
cleanly
- [ ] Run `task frontend:check` — verify typecheck + lint + test pass
- [ ] Run `task desktop:dev` — verify jlink builds are cached on second
run
- [ ] Verify CI passes on all workflows

---------

Co-authored-by: James Brunton <[email protected]>

docker/embedded/Dockerfile.fat

@@ -8,11 +8,16 @@ ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 # Stage 1: Build the Java application and frontend
 FROM gradle:9.3.1-jdk25 AS app-build
 
+ARG TASK_VERSION=3.49.1
 RUN apt-get update \
     && apt-get install -y --no-install-recommends curl ca-certificates \
     && update-ca-certificates \
     && curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \
+    && ARCH=$(dpkg --print-architecture) \
+    && curl -fsSL "https://github.com/go-task/task/releases/download/v${TASK_VERSION}/task_${TASK_VERSION}_linux_${ARCH}.deb" -o /tmp/task.deb \
+    && dpkg -i /tmp/task.deb \
+    && rm /tmp/task.deb \
     && rm -rf /var/lib/apt/lists/*
 
 # JDK 25+: --add-exports is no longer accepted via JAVA_TOOL_OPTIONS; use JDK_JAVA_OPTIONS instead