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/store/migrate_test.go

@@ -0,0 +1,116 @@
+package store
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestMigrate(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	// Create old structure
+	// WebDAV: files/<username>/
+	if err := os.MkdirAll(filepath.Join(tmpDir, "files", "alice"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(tmpDir, "files", "alice", "test.txt"), []byte("test"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// CalDAV: <username>/cal-<name>/
+	if err := os.MkdirAll(filepath.Join(tmpDir, "bob", "cal-work"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(tmpDir, "bob", "cal-work", "evt.ics"), []byte("calendar"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// CardDAV: <username>/card-<name>/
+	if err := os.MkdirAll(filepath.Join(tmpDir, "bob", "card-contacts"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(tmpDir, "bob", "card-contacts", "vcard.vcf"), []byte("card"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create store and migrate
+	st, err := NewStore(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := st.Migrate(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify WebDAV: <username>/files/
+	if _, err := os.Stat(filepath.Join(tmpDir, "alice", "files", "test.txt")); err != nil {
+		t.Errorf("alice files not migrated: %v", err)
+	}
+
+	// Verify CalDAV: <username>/calendars/<name>
+	if _, err := os.Stat(filepath.Join(tmpDir, "bob", "calendars", "work", "evt.ics")); err != nil {
+		t.Errorf("bob calendars not migrated: %v", err)
+	}
+
+	// Verify CardDAV: <username>/addressbooks/<name>
+	if _, err := os.Stat(filepath.Join(tmpDir, "bob", "addressbooks", "contacts", "vcard.vcf")); err != nil {
+		t.Errorf("bob addressbooks not migrated: %v", err)
+	}
+
+	// Verify old structure is gone
+	if _, err := os.Stat(filepath.Join(tmpDir, "files", "alice")); err == nil {
+		t.Error("old files directory not removed")
+	}
+	if _, err := os.Stat(filepath.Join(tmpDir, "bob", "cal-work")); err == nil {
+		t.Error("old cal- directory not removed")
+	}
+	if _, err := os.Stat(filepath.Join(tmpDir, "bob", "card-contacts")); err == nil {
+		t.Error("old card- directory not removed")
+	}
+}
+
+func TestMigrateIdempotent(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	// Create new structure
+	if err := os.MkdirAll(filepath.Join(tmpDir, "alice", "files"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(tmpDir, "alice", "files", "test.txt"), []byte("test"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	st, err := NewStore(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Run migration twice
+	if err := st.Migrate(); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Migrate(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify data still there
+	if _, err := os.Stat(filepath.Join(tmpDir, "alice", "files", "test.txt")); err != nil {
+		t.Errorf("data not preserved: %v", err)
+	}
+}
+
+func TestMigrateMissingDirectories(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	st, err := NewStore(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Should not error on empty directory
+	if err := st.Migrate(); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+}