v2.6.0: Usage Dashboard, per-provider tracking, OAuth file picker

- Usage Dashboard: visual cards with success rate bars, token stats, latency
- Per-model breakdown and error tracking per provider
- Proxy records usage-stats.json after every request
- Google OAuth: browse for client_secret.json instead of fixed path
- Auto-copies selected file to ~/.cache/codex-proxy/

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## v2.6.0 (2026-05-20)
+
+- **Usage Dashboard** — per-provider tracking with visual cards
+  - Request counts, success/failure rates, token usage, latency stats
+  - Color-coded success rate bars (green/yellow/red)
+  - Per-model breakdown showing request counts
+  - Last error and last used timestamp
+  - Sorted by most-used provider
+  - Refresh button for live updates
+- **Proxy usage tracking** — records every request to `usage-stats.json`
+- **Google OAuth**: browse for `client_secret.json` with file picker dialog
+  - No longer requires copying to a specific path manually
+  - Auto-copies selected file to `~/.cache/codex-proxy/`
+
 ## v2.5.1 (2026-05-20)
 
 - **Adaptive retry for transient errors** (429/502/503)