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
Refactor account selection into a strategy pattern with three options:
- Sticky: cache-optimized, stays on same account until rate-limited
- Round-robin: load-balanced, rotates every request
- Hybrid (default): smart distribution using health scores, token buckets, and LRU
The hybrid strategy uses multiple signals for optimal account selection:
health tracking for reliability, client-side token buckets for rate limiting,
and LRU freshness to prefer rested accounts.
Includes WebUI settings for strategy selection and unit tests.
Co-Authored-By: Claude <noreply@anthropic.com>
