test: cover grid filter OR/AND (extract pure fn + unit + real e2e)

The two filter-search cases were test.fixme placeholders (expect(true).toBe(true)),
so the OR/AND chip rules were untested.

- Extract the grid-filter predicate from feed/+page.svelte into a pure
  filterUploads(uploads, filters) in $lib/feed-filter (behavior-preserving) and
  unit-test it (9 cases): tag OR, user OR, tag+user AND, AND-excludes-partial,
  case-insensitive caption match, null caption.
- Replace the e2e fixmes with real tests that seed known captions/uploaders, switch
  to grid view, activate chips via the search suggestions, and count grid tiles:
  OR widens 1→2, AND narrows 2→1.

Frontend unit: 27 passing. filter-search e2e: 3 passing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/feed/+page.svelte

@@ -15,6 +15,7 @@
 	import { toast, toastError } from '$lib/toast-store';
 	import { pullToRefresh } from '$lib/actions/pull-to-refresh';
 	import { vibrate } from '$lib/haptics';
+	import { filterUploads } from '$lib/feed-filter';
 	import type { FeedUpload, FeedResponse, HashtagCount, DeltaResponse } from '$lib/types';
 
 	let uploads = $state<FeedUpload[]>([]);
@@ -149,14 +150,7 @@
 	// ── Filtered uploads for grid view ───────────────────────────────────────
 	let displayUploads = $derived.by(() => {
 		if (viewMode === 'list' || activeFilters.length === 0) return uploads;
-		const tags = activeFilters.filter((f) => f.type === 'tag').map((f) => f.value);
-		const users = activeFilters.filter((f) => f.type === 'user').map((f) => f.value);
-		return uploads.filter((u) => {
-			const cap = (u.caption ?? '').toLowerCase();
-			const passTag = !tags.length || tags.some((t) => cap.includes('#' + t));
-			const passUser = !users.length || users.includes(u.uploader_name);
-			return passTag && passUser;
-		});
+		return filterUploads(uploads, activeFilters);
 	});
 
 	// ─────────────────────────────────────────────────────────────────────────