Splash case picker + playable-mystery list

GET /api/mysteries returns only mysteries with an entrypoint (empties are
filtered out), and the splash now lists them as case files with a per-case
BEGIN / RESUME action instead of a single hardcoded New Game.

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

server/index.ts

@@ -373,6 +373,12 @@ app.delete('/api/admin/utterances/:id', requireAdmin, async (req, res, next) =>
   } catch (error) { next(error) }
 })
 
+// Play mode: the launchable case list for the splash picker (entrypoint required).
+app.get('/api/mysteries', async (_req, res, next) => {
+  try { res.json(await narrative.listPlayableMysteries()) }
+  catch (error) { next(error) }
+})
+
 // Narrative authoring: create a mystery and its NPC cast. The flow (cutscenes,
 // dialogue, levels) lives in the story graph, seeded separately.
 app.post('/api/mysteries', requireAdmin, async (req, res, next) => {