Add fullscreen preview, PWA support, and connect without a button

Fullscreen: a button on the preview pane fullscreens the live view, carrying
the countdown over as an overlay - fullscreen hides the entire control column,
so without it you're left staring at a picture with no idea when the next
frame lands.

Wake lock: held from app start until you switch it off, rather than only for
the duration of a sequence. You're usually mid-setup when the display would
otherwise sleep.

PWA: manifest, icons and a service worker, so it installs to a standalone
window and runs with no network. That matters more here than for most web
apps - the camera is on a USB cable, so this is fully functional in a field
with no signal.

The worker precaches the pinned unpkg dependencies at install rather than
leaving them to the runtime cache. On a first visit the worker isn't
controlling the page yet, so the app's own imports go straight to the network
and never reach the fetch handler; without the precache it looked cached but
died offline on its imports. Our own files are network-first so a redeploy
always wins, and the version-pinned CDN files are cache-first.

Connecting: no connect button. The app reaches for the camera on load, retries
every 1.5s, and listens for USB connect events, so switching the camera on
mid-wait attaches it with no click or reload. The exception is a browser that
has never been granted access to the device: Chrome will not open its WebUSB
chooser outside a user gesture, so that case still shows a one-off prompt.
After that the permission is remembered and getDevices() finds the camera with
no interaction.

tsconfig excludes sw.js, which runs in ServiceWorkerGlobalScope and reports
every worker global as undefined when checked against the DOM lib.

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

README.md

@@ -58,7 +58,9 @@ npm install
 npx serve .   # serve.json sets the COOP/COEP headers the WASM module needs
 ```
 
-Open the printed URL in Chrome, plug the camera in over USB, switch it on and pick it from the device picker.
+Open the printed URL in Chrome and plug the camera in. It connects on its own — there's no connect button. The one exception is the very first visit, which shows a "Choose camera" prompt: Chrome will not open its WebUSB device chooser without a click. After that the permission is remembered, and `navigator.usb.getDevices()` finds the camera with no interaction, so later visits go straight in. If the camera is off or asleep the page sits on "Looking for your camera…" and keeps retrying, connecting the moment it wakes up.
+
+It's also an installable PWA: install it from Chrome's address bar to get a standalone window, and it works with no network at all — the app shell and the pinned unpkg dependencies (including the ~2 MB WASM) are precached by a service worker. That matters here because the camera is on a USB cable, so the app is perfectly useful in a field with no signal.
 
 **Home screen** — only the things you change between runs:
 
@@ -66,6 +68,7 @@ Open the printed URL in Chrome, plug the camera in over USB, switch it on and pi
 - 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 ⛶ button on the preview for a fullscreen live view, with the countdown carried over as an overlay so you don't lose it
 - 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