perf: optimize container startup and CI pipeline

- Dockerfile: use Flutter tarball instead of git clone (~150MB vs 500MB+)
- Dockerfile: split into independent layers for better caching
- Dockerfile: use JRE instead of JDK, remove unnecessary packages
- devcontainer.json: remove venv/pip/claude from postCreateCommand
- ci.yml: remove redundant analyze/test in build-android job
- ci.yml: run lint and test in parallel (no dependency chain)

Estimated impact: 40min -> ~5-8min first build, ~2-3min cached

.gitea/workflows/ci.yml

@@ -47,11 +47,10 @@ jobs:
         run: dart format lib/ test/
 
 
-  # ── Stage 2: Unit & widget tests ─────────────────────────────────
+  # ── Stage 1: Unit & widget tests (parallel with lint) ────────────
   test:
     name: Test
     runs-on: ubuntu-latest
-    needs: [lint]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -129,12 +128,6 @@ jobs:
       - name: Generate stubs (lint gate support)
         run: python3 ci_stub_gen.py
 
-      - name: Analyze (gate before build)
-        run: flutter analyze --fatal-warnings
-
-      - name: Run tests (gate before build)
-        run: flutter test --reporter=expanded
-
       # Optional: run whitelabel build script to regenerate lib/generated/
       # Skip gracefully if theme.json is missing or script fails
       - name: Run whitelabel build script