"Printer is not connected. Call connect() first." fires due to no silent WebUSB reconnect and inconsistent fallback guards

[itsbkm]

Bug description

"Printer is not connected. Call connect() first." is tied for the #1 printer-related support complaint. It's thrown by the frontend WebUSB printer path any time a print is attempted while no WebUSB device handle is held — which is true after every app relaunch or page reload, and in several code paths where it fires even when a fully-configured backend (network/USB) printer exists.

Root cause

Thrown at frontend/src/lib/printer/PrinterService.ts:193:

if (!this.device || this._status !== 'connected') {
  throw new Error('Printer is not connected. Call connect() first.');
}

There are three separate defects that each independently trigger this:

1. No silent reconnect on startup. connect() (PrinterService.ts:76-158) only calls navigator.usb.requestDevice(), which requires a live user gesture and shows a picker every time. Nothing calls the non-prompting navigator.usb.getDevices() to silently re-acquire a device the user already granted permission for in a previous session. So this.device is null after every relaunch/reload until the user manually clicks "Connect" again in PrinterStatus.tsx (frontend/src/components/pos/PrinterStatus.tsx:84-96, only rendered via PosTopbar.tsx:49).

2. Inconsistent fallback guards across print functions in frontend/src/hooks/usePrinter.ts:

Function	Behavior when no backend printer / not connected
printBill (usePrinter.ts:107-216)	Falls back to executeBrowserPrint() (:162-167) — does not throw.
printTaxBill (usePrinter.ts:218-281)	No hardware-printer branch at all. Whenever printMethod !== 'browser', unconditionally calls printerService.print(bytes) (:275) — even on stores with a configured backend network/USB printer, because no /printers/print-tax-bill backend route exists.
printKot (usePrinter.ts:283-341)	Has a backend-hardware branch (:296-305), but its fallthrough (:307-314) calls printerService.print(bytes) directly with no isConnected check and no browser fallback, unlike printBill.

So printTaxBill and printKot throw this error in exactly the cases printBill handles gracefully.

3. A population race for hardwarePrinter. It's loaded only by refreshHardwarePrinter() (usePrinter.ts:80-92), invoked from usePrinterStatusSync() (usePrinter.ts:390-411), which only mounts on the POS page (via PrinterStatus.tsx) and Settings. It's explicitly excluded from persisted state (usePrinter.ts:375), so it's always null on fresh load and filled in asynchronously via GET /printers. Since KOT auto-print fires immediately on order placement (frontend/src/app/(dashboard)/pos/page.tsx:256-264, 530, 732, gated by autoPrintKot), a fast first order right after app start can race ahead of that fetch and hit the unguarded branch in defect #2.

Combined, this explains the complaint volume: it fires for (a) any WebUSB-only shop that reloaded/relaunched without re-clicking Connect, (b) every printTaxBill call on shops using tax/GST invoices regardless of printer type, and (c) intermittent KOT auto-print failures right after startup.

Proposed fix

1. In PrinterService.connect()/init, attempt a silent navigator.usb.getDevices() reconnect before falling back to the prompting requestDevice() flow, so a previously-granted device reattaches automatically on load.
2. Add the same hw-check + browser-fallback pattern that printBill already uses (usePrinter.ts:162-167) to printTaxBill and the printKot fallthrough branch, including adding a /printers/print-tax-bill backend route so tax invoices route through the backend printer path like bills do.
3. Hoist usePrinterStatusSync() to a layout-level provider (instead of only PrinterStatus.tsx) so hardwarePrinter is populated before the POS page can place an order, closing the startup race.

Environment

Affects WebUSB-configured printers on all platforms; also affects tax/GST-invoice printing on stores using backend network/USB printers.

[itsbkm]

Fix opened in #535 — PrinterService now silently re-attaches to a previously-granted WebUSB device on app start, printTaxBill/printKot fall back to browser printing instead of throwing, and printer status sync is hoisted to the dashboard layout to close the startup race. Also wired up the missing Electron main-process USB permission handlers (session.on('select-usb-device') + setDevicePermissionHandler) so the WebUSB device picker actually works in the packaged desktop app, not just a plain browser. Will merge once CI is green.

[itsbkm]

Merged in #535 (bbfe7b2).