fix(documents): sync filter display state with URL on navigation

Three root causes prevented filters from reflecting the URL after SvelteKit
client-side navigation:

1. +page.server.ts now resolves sender/receiver display names in parallel with
   the document search (UUID validation + silent 404 drop), so initialSenderName
   / initialReceiverName land in server data ready for the UI to use.

2. +page.svelte passes initialSenderName, initialReceiverName, and navKey
   (incremented via untrack on every navigation) down to SearchFilterBar.
   The untrack() prevents the effect from re-running due to its own navKey write.

3. SearchFilterBar forwards navKey as resetKey to each PersonTypeahead, which
   already had a void resetKey guard added in the previous commit.

Together these ensure that after navigating to /documents?senderId=<uuid> the
typeahead shows the person's display name, and clicking × reset clears it.

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

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

@@ -23,6 +23,8 @@ function makeData(overrides: Record<string, unknown> = {}) {
 		to: '',
 		senderId: '',
 		receiverId: '',
+		initialSenderName: '',
+		initialReceiverName: '',
 		tags: [],
 		sort: 'DATE',
 		dir: 'desc',
@@ -136,6 +138,22 @@ describe('documents page — URL building', () => {
 	});
 });
 
+// ─── Sender / receiver name display ──────────────────────────────────────────
+
+describe('documents page — sender/receiver display', () => {
+	it('pre-fills sender typeahead from initialSenderName when senderId filter is active', async () => {
+		render(Page, {
+			data: makeData({
+				senderId: '11111111-1111-1111-1111-111111111111',
+				initialSenderName: 'Max Mustermann'
+			})
+		});
+		// Advanced filters are auto-shown when senderId is set
+		const inputs = page.getByPlaceholder('Namen tippen...');
+		await expect.element(inputs.first()).toHaveValue('Max Mustermann');
+	});
+});
+
 // ─── Timeline density widget wiring (#385) ────────────────────────────────────
 
 describe('documents page — timeline density widget', () => {