feat(webui): add subscription tier and quota visualization

Backend:

CLAUDE.md

@@ -134,6 +134,7 @@ public/
 - **src/server.js**: Express server exposing Anthropic-compatible endpoints (`/v1/messages`, `/v1/models`, `/health`, `/account-limits`) and mounting WebUI
 - **src/webui/index.js**: WebUI backend handling API routes (`/api/*`) for config, accounts, and logs
 - **src/cloudcode/**: Cloud Code API client with retry/failover logic, streaming and non-streaming support
+  - `model-api.js`: Model listing, quota retrieval (`getModelQuotas()`), and subscription tier detection (`getSubscriptionTier()`)
 - **src/account-manager/**: Multi-account pool with sticky selection, rate limit handling, and automatic cooldown
 - **src/auth/**: Authentication including Google OAuth, token extraction, database access, and auto-rebuild of native modules
 - **src/format/**: Format conversion between Anthropic and Google Generative AI formats
@@ -148,6 +149,17 @@ public/
 - Session ID derived from first user message hash for cache continuity
 - Account state persisted to `~/.config/antigravity-proxy/accounts.json`
 
+**Account Data Model:**
+Each account object in `accounts.json` contains:
+- **Basic Info**: `email`, `source` (oauth/manual/database), `enabled`, `lastUsed`
+- **Credentials**: `refreshToken` (OAuth) or `apiKey` (manual)
+- **Subscription**: `{ tier, projectId, detectedAt }` - automatically detected via `loadCodeAssist` API
+  - `tier`: 'free' | 'pro' | 'ultra' (detected from `paidTier` or `currentTier`)
+- **Quota**: `{ models: {}, lastChecked }` - model-specific quota cache
+  - `models[modelId]`: `{ remainingFraction, resetTime }` from `fetchAvailableModels` API
+- **Rate Limits**: `modelRateLimits[modelId]` - temporary rate limit state (in-memory during runtime)
+- **Validity**: `isInvalid`, `invalidReason` - tracks accounts needing re-authentication
+
 **Prompt Caching:**
 - Cache is organization-scoped (requires same account + session ID)
 - Session ID is SHA256 hash of first user message content (stable across turns)
@@ -183,11 +195,13 @@ public/
 - **Architecture**: Single Page Application (SPA) with dynamic view loading
 - **State Management**: Alpine.store for global state (accounts, settings, logs)
 - **Features**:
-  - Real-time dashboard with Chart.js visualization
+  - Real-time dashboard with Chart.js visualization and subscription tier distribution
+  - Account list with tier badges (Ultra/Pro/Free) and quota progress bars
   - OAuth flow handling via popup window
   - Live log streaming via Server-Sent Events (SSE)
   - Config editor for both Proxy and Claude CLI (`~/.claude/settings.json`)
 - **Security**: Optional password protection via `WEBUI_PASSWORD` env var
+- **Smart Refresh**: Client-side polling with ±20% jitter and tab visibility detection (3x slower when hidden)
 
 ## Testing Notes
 
@@ -223,6 +237,12 @@ public/
 - `sleep(ms)` - Promise-based delay
 - `isNetworkError(error)` - Check if error is a transient network error
 
+**Data Persistence:**
+- Subscription and quota data are automatically fetched when `/account-limits` is called
+- Updated data is saved to `accounts.json` asynchronously (non-blocking)
+- On server restart, accounts load with last known subscription/quota state
+- Quota is refreshed on each WebUI poll (default: 30s with jitter)
+
 **Logger:** Structured logging via `src/utils/logger.js`:
 - `logger.info(msg)` - Standard info (blue)
 - `logger.success(msg)` - Success messages (green)
@@ -239,6 +259,9 @@ public/
 - `/api/claude/config` - Claude CLI settings
 - `/api/logs/stream` - SSE endpoint for real-time logs
 - `/api/auth/url` - Generate Google OAuth URL
+- `/account-limits` - Fetch account quotas and subscription data
+  - Returns: `{ accounts: [{ email, subscription: { tier, projectId }, limits: {...} }], models: [...] }`
+  - Query params: `?format=table` (ASCII table) or `?includeHistory=true` (adds usage stats)
 
 ## Maintenance