fix(search): prevent stale navigation from clobbering focused search input

The sync $effect on the home page unconditionally overwrote the local `q`
state with the URL value after every navigation. When users typed faster
than a navigation round-trip (debounce fires → goto() → data reloads),
the completed navigation wrote the stale URL value back into the input,
dropping the characters typed in the interim.

Guard the `q` assignment in the effect with a `qFocused` flag (set via
onfocus/onblur on the text input). Covers issue #34.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/page.svelte.spec.ts

@@ -164,6 +164,27 @@ describe('Home page – document list', () => {
 	});
 });
 
+// ─── Keystroke preservation (issue #34) ──────────────────────────────────────
+
+describe('Home page – search input keystroke preservation', () => {
+	it('does not overwrite the search input while the user is focused and stale data arrives', async () => {
+		const { rerender } = render(Page, { data: emptyData });
+
+		const input = page.getByPlaceholder('Suche in Titel, Inhalt, Ort...');
+
+		// User types "abc" — input is focused
+		await input.click();
+		await input.fill('abc');
+
+		// Simulate a navigation completing with stale data (q='a') while the user is still typing
+		await rerender({ data: { ...emptyData, filters: { ...emptyData.filters, q: 'a' } } });
+		await tick();
+
+		// Input must still show what the user typed, not the stale URL value
+		await expect.element(input).toHaveValue('abc');
+	});
+});
+
 // ─── Error state ──────────────────────────────────────────────────────────────
 
 describe('Home page – error state', () => {