feat: inline caption editing in the preview

[Alfredoalv13]

Summary

• Wires up the previously-unused inline caption editing engine (captionEditing.ts — updateCaptionCuesForEditedTarget, CaptionEditTarget) to real UI. It had full logic and test coverage but nothing in the app called it.
• Double-clicking the caption box in the editor preview enters an editable state: a textarea pre-filled with the visible caption block's text, auto-focused and selected.
• Enter commits (Shift+Enter inserts a newline), Escape or blur cancels.
• Playback auto-pauses when entering edit mode so the caption block being edited can't scroll out from under the textarea mid-edit.
• If the visible caption block changes out from under an open edit (e.g. a stray seek), the edit is safely dropped rather than committing against a stale target.

Test plan

• npx tsc -p tsconfig.json --noEmit — clean
• npx vitest --run — 819/819 passing
• Built, signed, and ran the packaged app directly with a real generated caption (real Whisper transcription on an existing recording)
• Double-clicked the caption box — textarea appeared pre-filled with the exact caption text, auto-focused
• Edited the text and pressed Enter — committed immediately, new text rendered in the preview
• Re-opened edit mode, typed a different draft, pressed Escape — draft discarded, prior committed text remained (not the escaped draft)