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/appSettings.ts

@@ -8,7 +8,7 @@ export interface AppSettings {
 
 const DEFAULTS: AppSettings = {
 	fileLoadLimit: 100,
-	tagRuleApplyToExisting: false,
+	tagRuleApplyToExisting: false
 };
 
 function load(): AppSettings {
@@ -25,4 +25,4 @@ export const appSettings = writable<AppSettings>(load());
 
 appSettings.subscribe((v) => {
 	if (browser) localStorage.setItem('app-settings', JSON.stringify(v));
-});
+});