v3: journal now shows the generated still for video dumps, like the hot/new feeds already did

The hot/new feeds go through FilePreview, which asks for
GET /api/thumbnails/:dumpId unconditionally on a video mime and lets VideoThumb
fall back to an icon if it 404s. The journal instead only recognised image files
and dump.thumbnailMime — and that field maps to custom_thumbnail_mime, i.e. a
thumbnail somebody uploaded by hand. Nothing on the Dump object advertises the
ffmpeg still the route generates on demand, so hasThumbnail() returned false,
mode degraded to text, and the card rendered a 🎬.

hasThumbnail() and JournalCard's thumbnail resolution now both treat a video
file as art, resolving to the same route. Since hasThumbnail() also gates grid
footprints, video dumps can now claim feature/tall/wide slots — the mosaic has
noticeably more image cards as a result, which is the point.

ThumbnailPlaceholder gains `glyph` and `seed`: a host without ffmpeg gets a 404
and lands on the placeholder, where the mime emoji says more about the dump than
an initial taken from its filename, and the hue is seeded from the filename
since there's no hostname to hash.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/utils/journalLayout.ts

@@ -17,7 +17,12 @@ export interface JournalEntry {
 
 /** A dump that can carry a real preview image (file image or rich thumbnail). */
 export function hasThumbnail(dump: Dump): boolean {
-  if (dump.kind === "file" && dump.fileMime?.startsWith("image/")) return true;
+  if (dump.kind === "file") {
+    const mime = dump.fileMime ?? "";
+    // Videos count: GET /api/thumbnails/:dumpId grabs a still with ffmpeg on
+    // first request and caches it, so there's art to show without any upload.
+    if (mime.startsWith("image/") || mime.startsWith("video/")) return true;
+  }
   if (dump.thumbnailMime) return true;
   return !!dump.richContent?.thumbnailUrl;
 }

src/utils/thumbnailTint.ts

@@ -43,13 +43,18 @@ function hostnameOf(url: string | undefined): string | undefined {
 /**
  * The page's own brand color when it declared a usable one, otherwise a hue
  * derived from its hostname — so every card gets a tint, including rows saved
- * before accent extraction existed.
+ * before accent extraction existed. File dumps have no hostname to hash and
+ * pass `seed` (their filename) instead.
  */
 export function tintFor(
-  { accentColor, url }: { accentColor?: string; url?: string },
+  { accentColor, url, seed }: {
+    accentColor?: string;
+    url?: string;
+    seed?: string;
+  },
 ): string {
   if (isSafeHex(accentColor)) return accentColor;
-  return hueFromString(hostnameOf(url) ?? url ?? "");
+  return hueFromString(seed ?? hostnameOf(url) ?? url ?? "");
 }
 
 /** The letter drawn when there's no favicon to show. */