fix(ui): add focus management to the shared Modal

[cavidelizade]

Summary

The shared Modal had role="dialog", aria-modal and Escape handling, but it never moved focus into the dialog, didn't trap Tab (keyboard focus could land on the page behind it), and didn't return focus to the trigger on close. Most modals reuse this primitive, so the gap repeated everywhere.

Linked issues

Closes #347

Type of change

• Bug fix (fix:)

Surface

• UI (apps/web/)

What changed

• Move focus to the first focusable field (or the dialog) when it opens.
• Trap Tab/Shift+Tab within the dialog.
• Restore focus to the previously focused element on close.
• Give the title a unique useId() for aria-labelledby instead of a shared static modal-title id (which would collide if two modals ever mounted).

Scope note

Issue #347 also lists the bespoke page-editor dropdowns and the inline confirm dialog in IssueDetailPage. This PR fixes the shared Modal, which is the widest-reaching piece; the dropdowns/inline dialog are worth a follow-up (they don't share this primitive).

Test plan

• npm run typecheck + npm run lint pass
• Manual: open a modal, focus starts inside it, Tab cycles within it, Escape closes it, and focus returns to the element that opened it

AI assistance

• AI tools were used, tool(s): Claude Code (Opus 4.8), commits carry a Co-Authored-By: trailer

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📥 Commits

Reviewing files that changed from the base of the PR and between 61bc903 and 332cf5e.

📒 Files selected for processing (1)

• apps/web/src/components/ui/Modal.tsx

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🧪 Generate unit tests (beta)

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