Add full nozzle rack metrics and per-slot hover cards for H2C printers

Capture 4 additional fields from MQTT nozzle data (max_temp, serial_number,
filament_color, filament_id) and surface them through REST/WebSocket APIs.
Add per-slot hover popover to the NozzleRackCard showing all metrics, filament
color backgrounds on slots, and i18n labels in all 4 locales.

frontend/src/api/client.ts

@@ -135,6 +135,10 @@ export interface NozzleRackSlot {
   nozzle_diameter: string;
   wear: number | null;
   stat: number | null;  // Nozzle status (e.g. mounted/docked)
+  max_temp: number;
+  serial_number: string;
+  filament_color: string;  // RGBA hex ("00000000" = no filament)
+  filament_id: string;
 }
 
 export interface PrintOptions {