Security: enforce trusted renderer origins (preload + navigation + IPC)

[gregpriday]

Summary

Implements defense-in-depth security controls to enforce trusted renderer origins across three critical layers: preload API exposure, navigation guards, and IPC sender validation. This prevents untrusted origins from accessing privileged APIs even if the main window is compromised via XSS or navigation attacks.

Closes #1889

Changes Made

• Add environment-aware origin allowlist (dev origins only in development mode)
• Implement preload API gating with main-frame check to prevent subframe privilege escalation
• Add global IPC sender validation via monkey-patch for comprehensive coverage
• Block same-window navigations/redirects to untrusted URLs
• Add comprehensive unit tests for origin validation with edge case coverage
• Update existing IPC handler tests with mock senderFrame for compatibility
• Configure test environment for development mode

Security Improvements

• Preload gating: window.electron API only exposed to main frame of trusted origins
• Navigation guards: will-navigate and will-redirect events block untrusted destinations
• IPC validation: All IPC handlers validate sender origin before execution (174 handlers protected)
• Environment-aware: Dev origins (localhost:5173, 127.0.0.1:5173) only trusted in development

Testing

• All existing tests pass
• New unit tests cover edge cases (malformed URLs, protocol normalization, query strings, etc.)
• TypeScript compilation successful