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>

frontend/src/lib/stores/auth.ts

@@ -31,4 +31,4 @@ authStore.subscribe((state) => {
 	}
 });
 
-export const isAuthenticated = derived(authStore, ($auth) => !!$auth.accessToken);
+export const isAuthenticated = derived(authStore, ($auth) => !!$auth.accessToken);