Extension: pre-roll countdown with cancel and skip

[sardorml]

Adds a 3-2-1 pre-roll countdown to the extension recorder: after the user hits Start Recording and every grant/picker settles, a countdown overlays the page with cancel (✕ / Esc) and skip controls before the first frame is captured.

Flow

panel "record" → SW startRecording → offscreen acquires streams (picker, webcam, mic)
                                            │
                                   onCountdown() ──→ SW runCountdown
                                            │          • injects overlay into the active tab
                                            │          • owns the 3s clock
                                   "go" / "cancel" ←── skip / ✕ / Esc end it early
                                            │
                              "go" → upload session starts   "cancel" → tracks stopped, status "cancelled"

Design notes

• Placement: the gate runs after all permissions and the native picker settle, and before the upload is created — cancelling costs nothing server-side (no multipart, no D1 row; reuses the existing picker-cancel cancelled path).
• The worker owns the clock; the page only displays it. Navigation mid-countdown can kill the overlay, but capture still starts on schedule. Skip/cancel ride the same raw runtime-message channel as recorderOverlayClosed.
• Overlay self-removes 150 ms before zero, so it never appears in the first recorded frames.
• Restricted pages (no injection possible) degrade to the current instant start.
• Overlay follows the content-script conventions: vanilla DOM, hand-scoped inline styles, serialized injected function — no framework in the page.

Verification

• pnpm typecheck, extension tests 57/57, wxt build (prod + dev) all green.
• Manual: dev build — countdown renders, numeral pulses per second; Esc, ✕ and skip all behave; recording starts cleanly after "1"; restricted pages record without the overlay.