[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/pubspec.yaml

@@ -21,15 +21,14 @@ dependencies:
   path: ^1.9.0
 
   # Models / serialization
-  freezed_annotation: ^2.4.0
   json_annotation: ^4.9.0
 
   # IDs
   ulid: ^2.0.0
 
-  # AI / on-device LLM (#215)
-  # flutter_gemma 는 OQ-1 (정확한 모델 URL + SHA) 확정 후 추가.
-  # v1은 LlmService 추상 + ModelLifecycle (파일/SHA/메타 관리) + Mock 까지 구현.
+  # AI / on-device LLM (#215, #218)
+  # #218 (v0.3.0): flutter_gemma 0.16.5 + Gemma 4 E2B (HF litert-community).
+  flutter_gemma: ^0.16.5
   crypto: ^3.0.0
   http: ^1.2.0
 
@@ -41,7 +40,6 @@ dev_dependencies:
   # Codegen
   drift_dev: ^2.18.0
   build_runner: ^2.4.0
-  freezed: ^2.5.0
   json_serializable: ^6.8.0
 
 flutter: