style(project): format Go with gofmt, set up Prettier for the frontend

Run gofmt -w across the backend, normalising the manually-aligned := blocks
to the gofmt standard. No code behaviour changes.

Add Prettier (+ prettier-plugin-svelte) to the frontend with the SvelteKit
default config (tabs, single quotes) so formatting is reproducible, then run
it over the whole tree. Add format / format:check npm scripts and a
.prettierignore (build output, generated schema.ts, static assets).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/handler/file_handler.go

@@ -89,16 +89,16 @@ type fileJSON struct {
 
 func toTagJSON(t domain.Tag) tagJSON {
 	j := tagJSON{
-		ID:          t.ID.String(),
-		Name:        t.Name,
-		Notes:       t.Notes,
-		Color:       t.Color,
+		ID:            t.ID.String(),
+		Name:          t.Name,
+		Notes:         t.Notes,
+		Color:         t.Color,
 		CategoryName:  t.CategoryName,
 		CategoryColor: t.CategoryColor,
-		CreatorID:   t.CreatorID,
-		CreatorName: t.CreatorName,
-		IsPublic:    t.IsPublic,
-		CreatedAt:   t.CreatedAt.Format(time.RFC3339),
+		CreatorID:     t.CreatorID,
+		CreatorName:   t.CreatorName,
+		IsPublic:      t.IsPublic,
+		CreatedAt:     t.CreatedAt.Format(time.RFC3339),
 	}
 	if t.CategoryID != nil {
 		s := t.CategoryID.String()
@@ -672,4 +672,4 @@ func parseUUIDs(strs []string) ([]uuid.UUID, error) {
 		ids = append(ids, id)
 	}
 	return ids, nil
-}
+}