perf(files): resolve breadcrumbs from a name cache, not N getFolder calls

Every navigation rebuilt the breadcrumb with one `GET /api/folders/{id}` per path
segment (a depth-D folder = D requests, each no-store) purely to label the trail.

Add an id→name cache, populated wherever a name is already known:
- every listing names its children, so `cacheFolder` records them, and
- `getFolder` records the folder it fetched.

`buildCrumbs` now reads names from the cache and only fetches the ids it hasn't
seen. During normal step-by-step navigation each ancestor was named by its
parent's listing, so the breadcrumb resolves with ZERO extra requests; only a
cold deep-link fetches its unknown ancestors (still in parallel). Folder renames
update the cache immediately so the trail stays correct.

The cache is a small LRU (cap 1000 — names are tiny) and is independent of the
listing cache (names survive a listing invalidation).

Validated: 3 new unit tests (listing populates child names, getFolder records,
rename overwrites) → 46 frontend tests green; npm run check; headless render of
the real files route (list + grid) — no errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Vb9QHmLZnEMzHz7MrFy6

frontend/src/routes/files/[...path]/+page.svelte

@@ -12,8 +12,10 @@
 		fetchFolderListing,
 		getCachedFolder,
 		getFolder,
+		getFolderName,
 		invalidateFolderCache,
 		moveFolder,
+		rememberFolderName,
 		renameFolder,
 		type FolderListing
 	} from '$lib/api/endpoints/folders';
@@ -125,15 +127,21 @@
 	}
 
 	async function buildCrumbs(segments: string[]): Promise<Array<{ id: string; name: string }>> {
-		// Names for each id in the trail; tolerate failures with a fallback label.
-		const metas = await Promise.all(
-			segments.map((id) =>
-				getFolder(id)
-					.then((f) => ({ id, name: f.name }))
-					.catch(() => ({ id, name: '…' }))
-			)
+		// Names come from the cache first (every listing names its children, so
+		// step-by-step navigation needs zero requests); only ids we've never seen
+		// — a cold deep-link's ancestors — are fetched, in parallel.
+		return Promise.all(
+			segments.map(async (id) => {
+				const known = getFolderName(id);
+				if (known !== undefined) return { id, name: known };
+				try {
+					const f = await getFolder(id);
+					return { id, name: f.name };
+				} catch {
+					return { id, name: '…' };
+				}
+			})
 		);
-		return metas;
 	}
 
 	// Bumped on every load; a stale in-flight response checks this before it
@@ -329,7 +337,10 @@
 		if (!name || name === current) return;
 		try {
 			if (kind === 'file') await renameFile(id, name);
-			else await renameFolder(id, name);
+			else {
+				await renameFolder(id, name);
+				rememberFolderName(id, name); // keep breadcrumbs current immediately
+			}
 			await reload();
 		} catch (e) {
 			errorToast(e);