config: Change useProxy default to false for general distribution

Change default behavior from Proxy mode to Direct API mode:
- useProxy defaultValue: true -> false
- General distribution: Users provide their own Gemini API key
- Development with Proxy: Use --dart-define=USE_PROXY=true

Rationale:
- Proxy mode requires Tailscale connection (not suitable for public distribution)
- Direct API mode works anywhere with internet connection
- Each user manages their own API quota

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/secrets.dart

@@ -23,11 +23,11 @@ class Secrets {
 
   /// AI Mode: Proxy(Home) vs Direct(Cloud)
   /// If false, connects directly to Google Gemini API (Works anywhere).
-  /// Release build: --dart-define=USE_PROXY=true
+  /// Development (with Tailscale): --dart-define=USE_PROXY=true
-  /// Development default: true (use local Synology proxy)
+  /// General distribution: false (each user provides their own Gemini API key)
   static const bool useProxy = bool.fromEnvironment(
     'USE_PROXY',
-    defaultValue: true,  // ← 開発環境ではProxy使用
+    defaultValue: false,  // ← 一般配布用: Direct API（各自がAPIキー設定）
   );