Add a spoken countdown between frames

A 🔊 toggle on the home screen counts you into each frame - "five, four,
three, two, one" - plus start and finish announcements. Useful when you're in
front of the camera rather than at the laptop. Uses the Web Speech API, so
it's local and needs no configuration.

The narrator runs off the intervalometer's state updates rather than a timer
of its own, so it can't drift away from what the sequence is doing; each
second is spoken at most once even though state arrives ~5x/sec.

The count is capped at one second under the interval, so a 3s interval says
"two, one" instead of talking over the previous frame, and a 1s interval stays
silent. A start delay isn't clamped, since that gap is whatever you set.

Countdown length, voice choice and frame-number announcements live in the
settings drawer; only the toggle is on the home screen.

Also:
- The toggle sits in the status card rather than the button row: three buttons
  don't fit a 400px column, and it wrapped onto its own flex line where it
  stretched to the wrong height.
- tsconfig excludes dist/, which build-dist.sh fills with copies of these same
  files and which otherwise gets type-checked twice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QK7PKpRVoy69Fpa32R8abb

README.md

@@ -65,6 +65,7 @@ Open the printed URL in Chrome, plug the camera in over USB, switch it on and pi
 - **Interval**, **frames** (or ∞ for open-ended) and **start delay**
 - Start/stop, plus a single-shot button for framing
 - Live countdown to the next frame, progress, estimated finish time, and a log of the last few events
+- A 🔊 toggle that speaks the countdown into each frame ("five, four, three, two, one") along with start and finish, for when you're in front of the camera rather than at the laptop
 - A read-only strip showing shutter / aperture / ISO / format / battery / shots remaining, so you can sanity-check exposure against the interval without opening anything
 - Pre-flight warnings when the interval is too tight for the current exposure and transfer time, when you're shooting RAW on a short interval, or when no output folder is set
 
@@ -73,6 +74,7 @@ Open the printed URL in Chrome, plug the camera in over USB, switch it on and pi
 - Where frames go: a folder on disk via the File System Access API, one-at-a-time downloads, or nothing at all if you're keeping them on the card. Frames are named after the moment they were taken — `20260801-172713_00007.JPG` in a sequence, `20260801-172713.JPG` for a single shot — so sorting by name is sorting by capture time. The camera's own `IMG_1234` is dropped: it wraps at 9999 and resets on a card format, so it can't order a long run.
 - Live view on/off, and whether it stays up between frames during a sequence (on by default — the feed drops only while each shot fires, since the camera needs live view down to take it, then recovers on its own)
 - Screen wake lock, so a backgrounded tab doesn't get its timers throttled mid-run
+- Voice countdown detail: how many seconds out to start counting (capped at one second under the interval so it never talks over the previous frame), which system voice to use, and whether to announce frame numbers
 - Bulb exposures, if the camera exposes `bulb` or `eosremoterelease`
 - The camera's own settings: shutter, aperture, ISO, image format, capture target, drive mode, and so on