fix: improve export filename and import confirmation UX (#232)

Export filename:
- Include username for multi-user/instance distinction
- Include timestamp with time (YYYYMMDD-HHMM) instead of date only
- Example: medassist-export-daniel-20260216-2108.json

Import confirmation:
- Show friendly 'Import Data?' dialog on empty instances instead of
  scary 'Replace All Data?' warning with danger button
- Only show destructive warning when there is existing data to replace
- Use primary button style for empty-state import

Closes #231

frontend/src/test/pages/SettingsPage.test.tsx

@@ -86,6 +86,7 @@ const createMockContext = (overrides = {}) => ({
 	handleImportConfirm: vi.fn(),
 	importResult: null,
 	setImportResult: vi.fn(),
+	meds: [],
 	...overrides,
 });