feat(documents): zoom-in tool for the timeline (#385)

Adds a zoom action that narrows the visible timeline range to the current
selection so the user can drill from year-level back into month-level
density. Zoom state lives in the URL (zoomFrom / zoomTo) so it survives
reload and is shareable.

- New `clipBucketsToRange(buckets, from, to)` helper applied before the
  >240-month year-aggregate decision, so a zoomed window flips back to
  month bars automatically when the clip narrows the range enough.
- `TimelineDensityFilter` gains `zoomFrom`, `zoomTo`, and `onzoomchange`
  props. Zoom button shown only when a selection exists and we aren't
  already zoomed; reset-zoom shown only when zoomed. Both placed in a
  shared right-edge action cluster alongside the × clear button.
- `+page.ts` reads zoomFrom/zoomTo from the URL and forwards them as
  props. `+page.svelte` extends FilterSnapshot + buildSearchParams, and
  triggerSearch accepts an optional zoom override so the onzoomchange
  callback can write the new pair (or clear them) atomically.
- 7 new component tests + 2 new page-integration tests cover the
  visibility rules and URL writes.
- 4 new unit tests for `clipBucketsToRange`.
- 3 new i18n keys (zoom in / zoom reset / drag aria-live) across de/en/es.

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

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

@@ -176,4 +176,55 @@ describe('documents page — timeline density widget', () => {
 		expect(url).toContain('from=1915-08-01');
 		expect(url).toContain('to=1915-08-31');
 	});
+
+	it('clicking the zoom-in button writes zoomFrom/zoomTo URL params', async () => {
+		const { goto } = await import('$app/navigation');
+		vi.mocked(goto).mockClear();
+
+		render(Page, {
+			data: makeData({
+				density: [
+					{ month: '1915-08', count: 3 },
+					{ month: '1915-09', count: 2 }
+				],
+				minDate: '1915-08-01',
+				maxDate: '1915-09-30',
+				from: '1915-08-01',
+				to: '1915-09-30'
+			})
+		});
+
+		const zoomBtn = document.querySelector('[data-testid="timeline-zoom-in"]') as HTMLButtonElement;
+		zoomBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(goto).toHaveBeenCalledOnce();
+		const [url] = vi.mocked(goto).mock.calls[0];
+		expect(url).toContain('zoomFrom=1915-08-01');
+		expect(url).toContain('zoomTo=1915-09-30');
+	});
+
+	it('clicking reset-zoom drops zoomFrom/zoomTo from the URL', async () => {
+		const { goto } = await import('$app/navigation');
+		vi.mocked(goto).mockClear();
+
+		render(Page, {
+			data: makeData({
+				density: [{ month: '1915-08', count: 3 }],
+				minDate: '1915-08-01',
+				maxDate: '1915-08-31',
+				zoomFrom: '1915-08-01',
+				zoomTo: '1915-08-31'
+			})
+		});
+
+		const resetBtn = document.querySelector(
+			'[data-testid="timeline-zoom-reset"]'
+		) as HTMLButtonElement;
+		resetBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+
+		expect(goto).toHaveBeenCalledOnce();
+		const [url] = vi.mocked(goto).mock.calls[0];
+		expect(url).not.toContain('zoomFrom=');
+		expect(url).not.toContain('zoomTo=');
+	});
 });