Modals don't manage focus consistently

[stultus]

Problem

The app's modals (Settings, Export, Metadata, Help, About) don't manage focus
consistently:

• Opening a modal sometimes leaves focus in the editor behind it.
• Tab doesn't trap inside the modal; users can tab back into the editor while the
  modal is still visible.
• Escape dismissal works for some modals and not others.
• On close, focus doesn't always return to the button that opened the modal.

Observed

• SettingsModal.svelte, ExportModal.svelte, MetadataModal.svelte all implement
  backdrop-click + Escape differently.
• No shared focus-trap utility; each modal re-invents its own behavior.

Proposed

• Introduce a shared modal wrapper component or action that:
  • Moves focus to the first focusable element on open.
  • Traps Tab/Shift-Tab within the modal.
  • Restores focus to the opener on close.
  • Handles Escape uniformly.
• Migrate all five modals to use it.

Acceptance

• Keyboard users can navigate every modal without accidentally focusing the editor.
• Escape and backdrop-click behave identically across modals.
• Focus returns to the trigger on dismiss.