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

config.example.json

@@ -32,7 +32,9 @@
   "persistTokenCache": false,
 
   "requestTimeoutMs": 300000,
+
   "maxAccounts": 10,
+  "_maxAccounts_comment": "Maximum number of Google accounts allowed (1-100). Default: 10.",
 
   "_profiles": {
     "development": {