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

README.md

@@ -362,6 +362,7 @@ While most users can use the default settings, you can tune the proxy behavior v
 - **Retry Logic**: Configure `maxRetries`, `retryBaseMs`, and `retryMaxMs`.
 - **Load Balancing**: Adjust `defaultCooldownMs` and `maxWaitBeforeErrorMs`.
 - **Persistence**: Enable `persistTokenCache` to save OAuth sessions across restarts.
+- **Max Accounts**: Set `maxAccounts` (1-100) to limit the number of Google accounts. Default: 10.
 
 Refer to `config.example.json` for a complete list of fields and documentation.