feat(ui): trap and restore focus in settings panel

[AlonsoFi]

Closes #61
Implements all focus management requirements from issue

• Focus trap: Tab and Shift+Tab cycle within the open panel, never reaching the page behind
• Escape key: closes the panel from any state
• Initial focus: close button receives focus when the panel opens
• Focus restoration: trigger button regains focus when the panel closes by any means
• Dialog semantics: role="dialog", aria-modal="true", aria-labelledby bound to the visible heading

Tests (Jest + RTL): 7 new a11y tests added, 21/21 passing
Lint: no warnings or errors
Build: clean production build, types valid

Keyboard flow verified:
Open panel → Tab through all controls → Tab past Done wraps to close button
→ Escape closes → gear trigger regains focus

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