fix(printer): surface real print errors and fix WebUSB connectivity

[itsbkm]

Summary

Fixes the two most-reported printer complaints (generic "Print failed" / "Printer is not connected" errors) and the manual-add name-mismatch bug, per investigation in #532, #533, #534.

• Print failures show generic "Print failed" message, hiding the real diagnostic already computed server-side #532 — /print-bill and /print-kot now return the backend's already-computed detail/failure_class instead of a generic message; surfaced in the POS support-error panel, orders-page reprint toast, and settings save/test toasts.
• Manually-added printer name can't match OS print queue, so test print silently fails #533 — loosened the printer-name validation regex (no shell-injection risk existed — names go through execFile array args or an env var, never a shell), trimmed whitespace on save, added a detected-printer autocomplete + mismatch warning on the manual-add form, and documented the exact-match requirement in docs/printers.md.
• "Printer is not connected. Call connect() first." fires due to no silent WebUSB reconnect and inconsistent fallback guards #534 — PrinterService silently re-attaches to a previously-granted WebUSB device on every app start instead of requiring a re-click; printTaxBill/printKot fall back to browser printing instead of throwing when no thermal transport is available; hoisted printer status sync to the dashboard layout to close a startup race. Also wired up the Electron main-process USB permission handlers (session.on('select-usb-device') + setDevicePermissionHandler) that were entirely missing — without them the WebUSB device picker could never resolve in the packaged desktop app.

Test plan

• npm run lint — 0 errors (pre-existing warnings only)
• npm run build — backend TypeScript compiles clean
• npm run build:frontend — Next.js build + type-check clean
• npm run i18n:check — translation parity across en/es/pt/fa passes
• npm run test:printer, test:print-parity, test:issue-134-routing, test:printer-migrations, test:printer-width-refresh, test:printer-fallback-and-popup-verification — all pass
• CI

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

The PR improves printer diagnostics, manual printer-name handling, browser fallback behavior, and WebUSB reconnection and authorization.

• Propagates detailed backend printing failures to POS, orders, and settings interfaces.
• Adds detected-printer name assistance and permits valid OS queue-name characters.
• Reconnects previously granted WebUSB printers and coordinates reconnects with printing and manual connections.
• Adds origin-scoped, user-mediated Electron USB permissions with safe persistent identities.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
main/usb-device-permissions.ts	Adds trusted-origin, explicit per-device USB authorization and safely limits serial-less approvals to the running session.
frontend/src/lib/printer/PrinterService.ts	Adds serialized device opening, silent reconnection, connection switching cleanup, and reconnect coordination.
frontend/src/hooks/usePrinter.ts	Waits for startup reconnection before routing print jobs and provides browser fallback when no thermal transport is available.
main/index.ts	Registers Electron USB permission handlers once against the stable application origin and shared session.
main/routes/printers.ts	Returns detailed print failures and broadens printer-name validation while trimming accidental surrounding whitespace.
frontend/src/app/(dashboard)/settings/page.tsx	Surfaces backend printer errors and adds detected-name autocomplete with exact-match guidance.

Sequence Diagram

sequenceDiagram
    participant UI as Printer UI
    participant PS as PrinterService
    participant USB as WebUSB
    participant Main as Electron permission handlers
    UI->>PS: Startup status sync
    PS->>USB: getDevices()
    USB->>Main: Check device permission
    Main-->>USB: Approve known device only
    USB-->>PS: Previously granted devices
    PS->>PS: Serialize openDevice()
    PS-->>UI: Connected status
    UI->>PS: Print
    PS->>PS: awaitPendingReconnect()
    alt Thermal transport connected
        PS->>USB: Transfer ESC/POS bytes
    else No thermal transport
        PS-->>UI: Browser print fallback
    end

Loading

_{Reviews (8): Last reviewed commit: "fix(printer): don't let serial-less USB ..." | Re-trigger Greptile}