feat(settings): confirm before deleting downloaded models

[karan68]

Summary

Deleting a downloaded model previously happened immediately on click, with no
confirmation. Since transcription/summary models are large (~600–700 MB) and must be
re-downloaded if removed by mistake, an accidental click was costly.

This PR gates model deletion behind the existing ConfirmationModal across all three
model managers — Whisper, Parakeet, and Summary (built-in AI) — without
changing any deletion logic or backend behaviour.

Addresses the confirmation gap called out in upstream [FEATURE] Delete downloaded
models in Settings (Zackriya-Solutions/meetily#597). The delete capability itself was
already implemented in Briefli; this adds the missing safety confirmation.

Why

• Prevents accidental, irreversible deletion of large models.
• Matches the pattern already used elsewhere in the app (meeting deletion) via the shared
  ConfirmationModal, so it's consistent and low-risk.

What changed

Per manager (WhisperModelManager.tsx, ParakeetModelManager.tsx, BuiltInModelManager.tsx):

• Added pendingDelete + isDeleting local state.
• Added confirmDelete() that wraps the existing deleteModel() — delete logic,
  toasts, list refresh, and the Rust commands are all unchanged.
• Delete actions (hover trash on downloaded models, and the Delete button on corrupted
  models) now open a confirmation dialog that names the model and the space it frees.
• While the delete runs, the dialog buttons disable and show a "Deleting…" state
  (isConfirming).
• Kept e.stopPropagation() so opening the dialog never selects/activates the model card.

No backend/Rust changes. No new dependencies.

Before / After

UI screenshots (Settings → Transcription / Summary → hover a downloaded model → Delete).

Before	After
Clicking the trash icon deleted the model instantly (no prompt).	A confirmation dialog appears before deletion.

Note: images to be attached in the GitHub UI — see PR comment.

Testing

• tsc --noEmit -p tsconfig.json → clean (exit 0).
• Bun test suite (tests/lib) → 63/63 pass.
• Manual checklist (to verify in-app):
  • Hover a downloaded model → trash → Cancel → nothing deleted.
  • Hover → trash → Delete → model removed, toast shown, disk space freed, list refreshes.
  • Delete the currently selected Whisper/Parakeet model → selection clears.
  • Corrupted model → Delete also routes through the dialog.
  • Repeat across all three managers (Whisper, Parakeet, Summary).

Scope / non-goals

• No changes to the Rust delete commands or engine delete_model (already correct).
• No progress bar — deletion is a single file/dir removal that completes instantly; the
  "Deleting…" state covers it.
• Did not change the pre-existing inconsistency where the built-in manager hides its
  trash icon for the currently-selected model (left as a separate follow-up).