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/lib/person/PersonTypeahead.svelte

@@ -47,7 +47,6 @@ let {
 
 // searchTerm must be both prop-derived AND locally writable (user typing), so $state +
 // $effect is the correct pattern here — writable $derived is read-only and won't work.
-// eslint-disable-next-line svelte/prefer-writable-derived
 let searchTerm = $state(initialName);
 
 // Sync display text when initialName changes OR when resetKey increments (navigation reset).