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

public/js/config/constants.js

@@ -69,6 +69,10 @@ window.AppConstants.VALIDATION = {
     MAX_WAIT_MIN: 60000,
     MAX_WAIT_MAX: 1800000,
 
+    // Max accounts range (1 - 100)
+    MAX_ACCOUNTS_MIN: 1,
+    MAX_ACCOUNTS_MAX: 100,
+
     // Rate limit dedup window (1 - 30 seconds)
     RATE_LIMIT_DEDUP_MIN: 1000,
     RATE_LIMIT_DEDUP_MAX: 30000,