feat: user-controlled session TTL with presets and custom duration

[OneStepAt4time]

Issue: #466

Implement user-controlled session TTL (Time To Live) with presets and optional custom duration.

Changes

• Add TTLSelector component with presets: 15m, 1h, 4h, 8h
• Add custom duration input for flexible TTL values
• Integrate TTL selector into session creation forms (single and batch modes)
• Wire ttl_seconds parameter to createSession/batchCreateSessions API calls
• Add comprehensive tests for TTLSelector component

Testing

• Unit tests for TTLSelector component with preset and custom input validation
• Dashboard type checking passes
• Component integration verified in CreateSessionModal

Closes #466

[aegis-gh-agent]

CI is failing: test (20) returned FAILURE. Please investigate and fix before I can approve. All other checks pass.

[aegis-gh-agent]

CI is failing: test (20) returned FAILURE. Please fix the test failures before I can approve.

[aegis-gh-agent]

Session TTL presets. Approved.

[aegis-gh-agent]

#993 was just merged and created 20+ new conflicts in #994 (same dashboard files, session.ts, package files). Please rebase #994 onto current main again and push.