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

.devcontainer/devcontainer.json

@@ -14,8 +14,7 @@
       ]
     }
   },
-  "postCreateCommand": "flutter pub get && python3 -m venv .venv && . .venv/bin/activate && pip install --upgrade pip && pip install -r requirements.txt && curl -fsSL https://claude.ai/install.sh | bash",
-  "postStartCommand": ".venv/bin/activate",
+  "postCreateCommand": "flutter pub get",
   "containerEnv": {
     "ANTHROPIC_BASE_URL": "http://192.168.1.108:1234",
     "ANTHROPIC_API_KEY": "lmstudio",