feat(M3): Dashboard-Builder — Edit-Modus, Drag&Drop, Palette, Tabs (#361)

- DashboardBuilder: @dnd-kit 12-Spalten-Flow-Grid (seed-konsistent), View/Edit-Schalter, Resize, Tab-Verwaltung, Dashboard-CRUD + Set-Default, Dirty-Save
- MiniAppHost ersetzt DashboardWidgetLoader (lazy Registry + settings-Props); Palette nur renderbare Apps (component-Filter)
- WidgetSettingsForm generisch aus settings_schema; Bestands-Widgets settings-fähig (RecentContacts: limit)
- api/miniapps.ts + api/dashboards.ts (TanStack-Query-Hooks, documents.ts-Muster)
- Dashboard.tsx = Builder-Host (StatCards/SystemMetrics bleiben bis M4); Legacy-Grid/Loader gelöscht, Geister-Test ersetzt
- Tests: Builder 13/13, Page 11/11, i18n de/en, tsc clean, build OK

frontend/src/i18n/locales/de.json

@@ -138,6 +138,30 @@
         "down": "Nicht verfügbar",
         "unknown": "Unbekannt"
       }
+    },
+    "builder": {
+      "edit": "Bearbeiten",
+      "save": "Speichern",
+      "create": "Neues Dashboard",
+      "delete": "Dashboard löschen",
+      "deleteConfirm": "Dashboard \"{{name}}\" wirklich löschen?",
+      "setDefault": "Als Standard",
+      "selectDashboard": "Dashboard auswählen",
+      "name": "Name",
+      "palette": "Widgets",
+      "paletteEmpty": "Keine Widgets verfügbar.",
+      "newTab": "Neuer Tab",
+      "addTab": "Tab hinzufügen",
+      "removeTab": "Tab entfernen",
+      "renameTab": "Tab umbenennen",
+      "tabName": "Tab-Name",
+      "widgetSettings": "Widget-Einstellungen",
+      "noSettings": "Keine Einstellungen verfügbar.",
+      "removeWidget": "Widget entfernen",
+      "wider": "Breiter",
+      "narrower": "Schmaler",
+      "taller": "Höher",
+      "shorter": "Niedriger"
     }
   },
   "companies": {

frontend/src/i18n/locales/en.json

@@ -138,6 +138,30 @@
         "down": "Down",
         "unknown": "Unknown"
       }
+    },
+    "builder": {
+      "edit": "Edit",
+      "save": "Save",
+      "create": "New dashboard",
+      "delete": "Delete dashboard",
+      "deleteConfirm": "Really delete dashboard \"{{name}}\"?",
+      "setDefault": "Set as default",
+      "selectDashboard": "Select dashboard",
+      "name": "Name",
+      "palette": "Widgets",
+      "paletteEmpty": "No widgets available.",
+      "newTab": "New tab",
+      "addTab": "Add tab",
+      "removeTab": "Remove tab",
+      "renameTab": "Rename tab",
+      "tabName": "Tab name",
+      "widgetSettings": "Widget settings",
+      "noSettings": "No settings available.",
+      "removeWidget": "Remove widget",
+      "wider": "Wider",
+      "narrower": "Narrower",
+      "taller": "Taller",
+      "shorter": "Shorter"
     }
   },
   "companies": {