feat(config): add configurable max accounts limit (#156)

Adds `maxAccounts` configuration parameter to control the maximum number of Google accounts.

**Changes:**
- New config field `maxAccounts` (default: 10, range: 1-100)
- Settings page: slider control for adjusting limit
- Accounts page: counter badge (e.g., "8/10") with visual feedback
- Add button disabled when limit reached
- Server-side validation on account creation

**Breaking Changes:** None

src/config.js

@@ -15,6 +15,7 @@ const DEFAULT_CONFIG = {
     persistTokenCache: false,
     defaultCooldownMs: 10000,  // 10 seconds
     maxWaitBeforeErrorMs: 120000, // 2 minutes
+    maxAccounts: 10, // Maximum number of accounts allowed
     modelMapping: {},
     // Account selection strategy configuration
     accountSelection: {