feat: add Gemini CLI session log usage tracking

Parse ~/.gemini/tmp/*/chats/session-*.json for precise per-message
token data (input/output/cached/thoughts). Integrates with existing
background sync and manual sync button alongside Claude and Codex.

src-tauri/src/services/mod.rs

@@ -11,6 +11,7 @@ pub mod provider;
 pub mod proxy;
 pub mod session_usage;
 pub mod session_usage_codex;
+pub mod session_usage_gemini;
 pub mod skill;
 pub mod speedtest;
 pub mod stream_check;