[Developer] #218 Real Gemma 4 E2B integration via flutter_gemma 0.16.5

Implements the OQ-1 follow-up to #215 v0.2.0: replace the placeholder
GemmaLlmService stub with a real flutter_gemma 0.16.5 backend driving
Gemma 4 E2B (litert-community/gemma-4-E2B-it-litert-lm, 2.41GB).

Highlights:
- GemmaLlmService.load → FlutterGemma.initialize + installModel.fromFile +
  getActiveModel; idempotent + FileSystemException on missing file.
- generateStructured uses Gemma 4 native function calling via
  createChat(tools: [Tool(...)], toolChoice: required). Stream parsed by
  collectFunctionCall — first FCR wins, ParallelFCR first-call wins,
  TextResponse/ThinkingResponse skipped, errors sanitized to prevent
  prompt leakage.
- main.dart wires _LazyLlmService adapter that resolves to GemmaLlmService
  when ModelLifecycle reports ready, MockLlmService otherwise.
- ai_providers.dart pins real model URL + SHA-256 (181938...39a63c).
- F2 hardening: ModelLifecycle.purge wraps each delete + meta remove in
  try/catch so a single OS-level flake cannot block opt-out.
- Android: INTERNET / FOREGROUND_SERVICE / POST_NOTIFICATIONS permissions
  + R8 proguard-rules.pro keeping MediaPipe / LiteRT / TFLite / protobuf
  JNI entry points (release builds otherwise crash on first inference).

Design-First: fn-gemma_llm_service.md updated to v2 — §C
(_appendSchemaInstruction) deprecated after reading flutter_gemma
0.16.5 source (Gemma 4 SDK injects tool declarations via template;
prompt-side append would double-wrap).

Tests:
- 10 new unit tests for collectFunctionCall covering all 8 fn-spec
  cases + 2 ParallelFunctionCallResponse paths.
- All 81 existing tests still pass.
- flutter analyze: 0 issues.

Refs #218

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/lib/ui/screens/settings_screen.dart

@@ -51,7 +51,7 @@ class _AiSection extends ConsumerWidget {
         SwitchListTile(
           title: const Text('AI 도움 켜기'),
           subtitle: const Text(
-            'Gemma 4 E2B 모델 ≈ 1.5GB. 모든 처리는 단말에서 일어납니다.',
+            'Gemma 4 E2B 모델 ≈ 2.4GB. 모든 처리는 단말에서 일어납니다.',
           ),
           value: optIn,
           onChanged: (v) async {
@@ -125,9 +125,10 @@ class _AiSection extends ConsumerWidget {
               style: TextStyle(fontWeight: FontWeight.w600),
             ),
             SizedBox(height: 12),
-            _Bullet('파일 크기: 약 1.5GB'),
+            _Bullet('파일 크기: 약 2.4GB'),
             _Bullet('WiFi 연결을 권장합니다'),
             _Bullet('모든 처리는 단말에서만 일어나며, 입력 텍스트는 외부로 전송되지 않습니다'),
+            _Bullet('Gemma 이용약관(ai.google.dev/gemma/terms)에 동의합니다'),
           ],
         ),
         actions: [
@@ -156,7 +157,7 @@ class _AiSection extends ConsumerWidget {
             Text('모델 파일이 단말에서 삭제됩니다.'),
             SizedBox(height: 8),
             Text(
-              '약 1.5GB 의 저장공간이 확보돼요.',
+              '약 2.4GB 의 저장공간이 확보돼요.',
               style: TextStyle(fontSize: 13, color: Colors.grey),
             ),
             SizedBox(height: 4),