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.

cmd/server/main.go

@@ -51,6 +51,13 @@ func main() {
 		os.Exit(1)
 	}
 
+	// ---- Run migration if data directory needs restructuring ----
+	if err := st.Migrate(); err != nil {
+		logger.Error("migration failed", "error", err)
+		os.Exit(1)
+	}
+	logger.Info("data directory migration completed")
+
 	// ---- Database (users, calendars, address books, sharing) ----
 	dbPath := filepath.Join(cfg.Storage.DataDir, "nidus.db")
 	dbase, err := db.Open(dbPath)