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.ts

@@ -19,5 +19,8 @@ export const load: PageLoad = async ({ url, fetch, data }) => {
 	};
 
 	const density = await fetchDensity(fetch, view, isDesktop, filters);
-	return { ...data, ...density };
+	const zoomFrom = url.searchParams.get('zoomFrom');
+	const zoomTo = url.searchParams.get('zoomTo');
+
+	return { ...data, ...density, zoomFrom, zoomTo };
 };