feat(documents): paginate search with a Pagination control

Frontend side of the /documents pagination work. The page.server.ts load
reads ?page= from the URL, forwards page+size=50 to the backend, and
exposes the new totalElements/pageNumber/pageSize/totalPages fields on
`data`. +page.svelte renders a <Pagination> component below the result
list; buildPageHref preserves every filter param and only updates page.
The existing triggerSearch debounce flow intentionally drops `page`
when any filter changes, so filter edits reset to page 0 automatically.

<Pagination> uses plain <a href> links (not goto) so SvelteKit's default
scroll restoration scrolls new pages to the top — the expected senior-UX
behaviour. Decorative chevrons wrapped in aria-hidden spans, 44px touch
targets, focus-visible ring, stacks vertically under 640px. The control
hides itself when totalPages ≤ 1.

Test coverage: 9 cases on Pagination (label, aria-current, prev/next
enable/disable, makeHref invocation, decorative chevron, touch target),
plus a filter-reset assertion on +page.svelte (page 5 → edit q →
goto URL must drop page=). Adds i18n keys in de/en/es. Manual edit to
api.ts pending a post-merge npm run generate:api against a rebuilt
dev backend. (#315)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/de.json

@@ -806,5 +806,9 @@
 	"chronik_load_more": "Mehr laden",
 	"chronik_loading": "Lädt …",
 	"chronik_load_more_announcement": "{count} weitere Einträge geladen",
-	"chronik_view_all": "Alle Aktivitäten →"
+	"chronik_view_all": "Alle Aktivitäten →",
+	"pagination_prev": "Zurück",
+	"pagination_next": "Weiter",
+	"pagination_page_of": "Seite {page} von {total}",
+	"pagination_nav_label": "Seitennavigation"
 }