frontend: keyboard combobox for appointments + clickable edit sheet

- new reusable <Combobox> (Base UI Autocomplete): arrow-key + Enter navigation
- New Appointment dialog: patient search and provider are now searchable
  comboboxes (provider sourced from /api/staff/providers) instead of a hand-
  rolled dropdown and a free-text input
- appointment rows are clickable and open an <AppointmentDetailSheet> to edit
  date/time/type/provider/status or delete; "Added by AI" badge shown on rows
  and in the sheet for source="ai" records

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/i18n/locales/en/translation.json

@@ -238,6 +238,7 @@
       "change": "Change",
       "searchPlaceholder": "Search name or file number",
       "noPatients": "No patients found.",
+      "noProviders": "No doctors found.",
       "date": "Date",
       "time": "Time",
       "type": "Type",
@@ -249,6 +250,25 @@
       "pickPatientBody": "Search and select a patient first.",
       "addedTitle": "Appointment added"
     },
+    "sheet": {
+      "title": "Appointment",
+      "editHint": "Click any appointment to edit — this is enabled.",
+      "date": "Date",
+      "time": "Time",
+      "type": "Type",
+      "provider": "Provider",
+      "status": "Status",
+      "save": "Save changes",
+      "saving": "Saving…",
+      "delete": "Delete",
+      "savedTitle": "Appointment updated",
+      "savedBody": "Your changes were saved.",
+      "saveFailedTitle": "Couldn't save",
+      "saveFailedBody": "Please try again.",
+      "deletedTitle": "Appointment deleted",
+      "deleteFailedTitle": "Couldn't delete",
+      "deleteFailedBody": "Please try again."
+    },
     "calendarDialog": {
       "today": "Today",
       "appointmentCount_one": "{{count}} appointment",