Modals have no focus trap and do not restore focus on close

[yash-sangwan]

The hand-rolled modals share the same accessibility gaps:

• no role="dialog" and no aria-modal="true", so screen readers do not announce them as dialogs
• no focus trap, so Tab walks straight out of the modal into the page behind it while the backdrop still blocks the mouse
• focus is never returned to the trigger on close, so keyboard users land back at the top of the document
• the preview modal never moves focus into itself at all
• the create-folder dialog puts its Escape handler on an inner div, so Escape only works while focus happens to be inside it
• the preview modal backdrop is a clickable <div> with no keyboard equivalent

@radix-ui/react-dialog is already a dependency and shared/components/ui/dialog.tsx wraps it, so most of this is solved by using what we have.

Where:

• frontend/src/components/file-preview/file-preview-modal.tsx
• frontend/src/features/dashboard/components/ui/dialogs/create-folder-dialog.tsx
• frontend/src/features/dashboard/components/views/search/advance-search-sheet/advanced-search-sheet.tsx
• frontend/src/features/upload/components/duplicate-dialog.tsx
• frontend/src/shared/components/ui/dialog.tsx

What to do:

• Port these onto the Radix dialog primitive, which gives focus trap, focus restore, Escape, and the ARIA wiring
• Where a custom modal has to stay, add role="dialog", aria-modal, aria-labelledby, focus-on-open, and focus-restore-on-close
• Move Escape handling to a document listener rather than a element-scoped one