feat: printer beep support for kitchen tickets

Adds configurable buzzer control so the printer alerts kitchen staff
on each new ticket print.

Backend:
  - printer_service.py: ESC 0x07 beep command fired before paper cut,
    reads print.beep_on_ticket / print.beep_pattern settings; supports
    single/double/triple/long presets and a custom:n1:n2:n3 format
  - settings.py: registers the two new print.beep_* settings with
    defaults (beep_on_ticket=true, beep_pattern=double)
  - system.py: POST /api/system/printers/test-beep endpoint for
    live testing from the dashboard

Frontend (PrintFontsTab):
  - BeepSection component with preset grid, custom n1/n2/n3 inputs,
    and a live test-beep button targeting any configured printer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/routers/settings.py

@@ -37,6 +37,9 @@ VALID_SETTINGS = {
     "print.font_ingredient":   "Font for removed ingredient lines (- marker): SIZE:BOLD:CAPS",
     "print.font_item_note":    "Font for per-item note lines: SIZE:BOLD:CAPS",
     "print.font_order_note":   "Font for order-level notes: SIZE:BOLD:CAPS",
+    # Beep settings
+    "print.beep_on_ticket":    "Play beep when a kitchen ticket prints: 'true' | 'false'",
+    "print.beep_pattern":      "Beep pattern: 'single' | 'double' | 'triple' | 'long' | 'custom:n1:n2:n3'",
 }
 
 DEFAULTS = {
@@ -63,6 +66,8 @@ DEFAULTS = {
     "print.font_ingredient":   "0:0:0",
     "print.font_item_note":    "0:0:0",
     "print.font_order_note":   "0:1:0",
+    "print.beep_on_ticket":    "true",
+    "print.beep_pattern":      "double",
 }