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/db/postgres/file_repo.go

@@ -142,7 +142,7 @@ func makeCursor(r fileRow, sort, order string) fileCursor {
 		}
 	case "mime":
 		val = r.MIMEType
-	// "created": val is empty; f.id is the sort key.
+		// "created": val is empty; f.id is the sort key.
 	}
 	return fileCursor{Sort: sort, Order: order, ID: r.ID.String(), Val: val}
 }
@@ -569,7 +569,7 @@ func (r *FileRepo) List(ctx context.Context, params domain.FileListParams) (*dom
 			cursorVal = av.OriginalName
 		case "mime":
 			cursorVal = av.MIMEType
-		// "created": cursorVal stays ""; cursorID is the sort key.
+			// "created": cursorVal stays ""; cursorID is the sort key.
 		}
 		hasCursor = true
 		isAnchor = true