Fix Connect/Disconnect button behavior across all workflows

[makermelissa-piclaw]

Closes #373.

Fixes the Connect/Disconnect button getting stuck in a wrong state across USB, Web, and BLE workflows when the underlying connection is interrupted, plus the second-click UX.

What was broken

From the issue (and tyeth's repro):

Uncaught (in promise) NetworkError: The device has been lost.
usb.js:277  Uncaught (in promise) NetworkError: The device has been lost.
script.js:592  Uncaught (in promise) NetworkError: The device has been lost.

When the board was unplugged (or the browser lost its USB grant, or the WiFi link dropped):

1. The editor didn't always notice it was disconnected → the Disconnect button stayed visible and did nothing.
2. Even when it did notice, clicking Connect afterwards re-prompted for a workflow instead of reconnecting to the same one.
3. Once the user picked a workflow, there was no way to back out to the chooser without closing/reopening the dialog.

Changes

1. USB workflow — handle "device has been lost" cleanly (js/workflows/usb.js)

The unhandled rejections traced back to two places:

• _readSerialLoop().catch(...) called onDisconnected() with the default reconnect=true. That re-entered connect() → connectToSerial() → _switchToDevice() → device.open() on a dead device, which threw another NetworkError — the one that surfaced as usb.js:277.
• serialTransmit() had three sequential await this.writer.ready / .write / .ready calls; only two had .catch() handlers. The trailing await this.writer.ready was the source of the script.js:592 rejection (terminal onData → serialTransmit).

Fixes:

• Read-loop catch now passes reconnect=false.
• serialTransmit() is wrapped in a single try/catch; any failure is treated as a disconnect (onDisconnected(null, false)), so we don't keep poking a dead writer.
• onDisconnected() is now idempotent (a small _isDisconnecting flag) — the WebSerial disconnect event and the read-loop rejection both fire on a yank, and previously both called us.
• If any of reader.cancel() / writer.releaseLock() / serialDevice.close() themselves throw NetworkError (because the device is gone), safeReconnect is forced to false so we don't fall back through the bad reconnect path.

2. Web workflow — notice silent drops (js/workflows/web.js)

• websocket.onerror is now wired and fires onDisconnected(event, false). Previously only onclose was handled, which doesn't reliably fire when the network goes away mid-session.
• serialTransmit() checks websocket.readyState and bails / disconnects if it's not OPEN, instead of throwing into a try/catch and silently logging.
• The PING_INTERVAL_MS keepalive watchdog (_checkConnection) was implemented but commented out — it's now re-enabled in onConnected(). A timed-out ping calls onDisconnected(null, false) so the UI flips back to Connect.

3. Connect remembers the last workflow (js/script.js)

• A new webeditor.lastBackend localStorage entry is updated whenever loadWorkflow() actually loads a workflow.
• checkConnected() now prefers that backend and goes straight to its connect dialog instead of always opening the workflow chooser.
• A new chooseAndShowConnect() is exposed via the workflow init params (chooseConnectionFunc) so the connect dialogs can re-prompt for a workflow when the user wants to switch.

4. "Choose a different workflow" link in each connect dialog

• index.html adds a <p class="connect-back-row"><a class="connect-back">… Choose a different workflow</a></p> to the BLE / Web / USB connect modals.
• Workflow._wireBackToChooser(modal) (new helper on the base class) attaches the click handler idempotently. Each subclass's showConnect() calls it after getModal().
• A small block in sass/layout/_layout.scss styles the back link without shifting the existing modal layout.

Testing notes

I'd suggest verifying:

• USB: connect → unplug board → Disconnect button flips to Connect, no NetworkError spam in console. Click Connect → goes straight back to the USB connect dialog.
• Web: connect to a board over WiFi → kill the network briefly → UI flips to Connect within ~5–10 s (ping watchdog).
• USB → Disconnect → click Connect → USB dialog re-opens; click "Choose a different workflow" → workflow chooser appears.
• BLE: a BLE-specific reconnect path was deliberately left alone (the existing gattserverdisconnected handler + the dialog's Reconnect button are how BLE was already behaving). Happy to follow up with a BLE-specific tightening if you'd like.

Notes for review

• The webeditor.lastBackend value is only used as a hint — it never auto-attempts a hardware connection on its own; it just routes the user to the matching connect dialog. URL-supplied ?backend=… still wins (handled in getBackend()).
• _isDisconnecting is reset in a finally, so a future failure inside the base super.onDisconnected() won't permanently jam the workflow.
• No new dependencies. npm run build passes locally.

[makermelissa]

Tested and working perfectly.