chore: rename MedAssist to MedAssist-ng in all frontend UI (#74)

Update all visible text from 'MedAssist' to 'MedAssist-ng':
- Auth page titles (login, register)
- Loading/error/initializing states
- SharedSchedule page (loading, expired, error, footer)
- AboutModal fallback text
- i18n strings for export file validation (EN/DE)
- Related test expectations

frontend/src/i18n/en.json

@@ -412,7 +412,7 @@
     "importSuccess": "Data imported successfully",
     "importSuccessDetails": "Imported: {{medications}} medications, {{doses}} doses, {{shares}} share links",
     "importError": "Failed to import data",
-    "invalidFile": "Invalid file format. Please select a valid MedAssist export file.",
+    "invalidFile": "Invalid file format. Please select a valid MedAssist-ng export file.",
     "downloadFilename": "medassist-export"
   },
   "refill": {