refactor: unify data directory structure

Unify user data directory from fragmented layout to consistent nested format:

- Move WebDAV from: data/files/<username> → data/<username>/files/
- Move CalDAV from: data/<username>/cal-<name> → data/<username>/calendars/<name>
- Move CardDAV from: data/<username>/card-<name> → data/<username>/addressbooks/<name>

Changes:
- internal/store/store.go: Update collectionPath() to map collection names
- internal/store/migrate.go: Add idempotent Migrate() method
- internal/store/migrate_test.go: Comprehensive migration tests
- internal/webdav/handler.go: Use new unified path structure
- cmd/server/main.go: Auto-run migration on startup
- tools/nidusctl/main.go: Add migrate subcommand
- Update tests to verify new structure

URL endpoints unchanged - only on-disk structure modified. All tests pass.

internal/webdav/handler_test.go

@@ -54,10 +54,10 @@ func TestPerUserIsolationAndPrefix(t *testing.T) {
 		t.Fatalf("expected bob to get 404 for alice's file, got %d", bobRec.Code)
 	}
 
-	// Confirm the file physically landed under dataDir/files/alice/, not
+	// Confirm the file physically landed under dataDir/alice/files/, not
 	// nested under an extra files/files/... path.
-	if _, err := os.Stat(dir + "/files/alice/note.txt"); err != nil {
-		t.Fatalf("expected file at dataDir/files/alice/note.txt: %v", err)
+	if _, err := os.Stat(dir + "/alice/files/note.txt"); err != nil {
+		t.Fatalf("expected file at dataDir/alice/files/note.txt: %v", err)
 	}
 }