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/lib/document/timeline.spec.ts

@@ -8,7 +8,8 @@ import {
 	buildDensityUrl,
 	aggregateToYears,
 	selectionBoundaryFrom,
-	selectionBoundaryTo
+	selectionBoundaryTo,
+	clipBucketsToRange
 } from './timeline';
 
 describe('monthBoundaryFrom', () => {
@@ -141,6 +142,37 @@ describe('aggregateToYears', () => {
 	});
 });
 
+describe('clipBucketsToRange', () => {
+	const buckets = [
+		{ month: '1915-08', count: 5 },
+		{ month: '1915-09', count: 2 },
+		{ month: '1915-10', count: 8 },
+		{ month: '1915-11', count: 3 }
+	];
+
+	it('returns the original buckets when range bounds are null', () => {
+		expect(clipBucketsToRange(buckets, null, null)).toBe(buckets);
+	});
+
+	it('keeps only buckets whose month falls within the range', () => {
+		expect(clipBucketsToRange(buckets, '1915-09-01', '1915-10-31')).toEqual([
+			{ month: '1915-09', count: 2 },
+			{ month: '1915-10', count: 8 }
+		]);
+	});
+
+	it('returns an empty array when the range excludes everything', () => {
+		expect(clipBucketsToRange(buckets, '1916-01-01', '1916-12-31')).toEqual([]);
+	});
+
+	it('treats partial dates correctly when bounds cross month boundaries', () => {
+		expect(clipBucketsToRange(buckets, '1915-09-15', '1915-10-15')).toEqual([
+			{ month: '1915-09', count: 2 },
+			{ month: '1915-10', count: 8 }
+		]);
+	});
+});
+
 describe('selectionBoundaryFrom / To', () => {
 	it('handles month labels (YYYY-MM)', () => {
 		expect(selectionBoundaryFrom('1915-08')).toBe('1915-08-01');