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/service/category_service.go

@@ -10,14 +10,14 @@ import (
 	"tanabata/backend/internal/port"
 )
 
-const categoryObjectType          = "category"
-const categoryObjectTypeID int16  = 3 // third row in 007_seed_data.sql object_types
+const categoryObjectType = "category"
+const categoryObjectTypeID int16 = 3 // third row in 007_seed_data.sql object_types
 
 // CategoryParams holds the fields for creating or patching a category.
 type CategoryParams struct {
 	Name     string
 	Notes    *string
-	Color    *string         // nil = no change; pointer to empty string = clear
+	Color    *string // nil = no change; pointer to empty string = clear
 	Metadata json.RawMessage
 	IsPublic *bool
 }
@@ -176,4 +176,4 @@ func (s *CategoryService) Delete(ctx context.Context, id uuid.UUID) error {
 func (s *CategoryService) ListTags(ctx context.Context, categoryID uuid.UUID, params port.OffsetParams) (*domain.TagOffsetPage, error) {
 	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.tags.ListByCategory(ctx, categoryID, params)
-}
+}