feat: name downloaded file after the video title and add a back-to-home link

The download endpoint served files under their internal uuid-based
storage name; it now builds the Content-Disposition filename from the
probed video title (persisted on submit) with the original extension,
falling back to the old behavior when no title is available. The
status page also gets an always-visible link back to the homepage.

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

prisma/migrations/20260811134726_add_video_title/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE `Download` ADD COLUMN `title` TEXT NULL;

prisma/schema.prisma

@@ -19,6 +19,7 @@ model Download {
   clipEnd     Int?
   audioQuality String?
   extraArgs   String?   @db.Text
+  title       String?   @db.Text
 
   filePath    String?   @db.Text
   fileName    String?