feat: implement Session Mode UI for Fund Once, Ask Many interactions

- Fix slideStyles used-before-declaration error in onboarding.tsx that
  blocked typecheck (introduced by #79). Moved slideStyles const above
  the slides array that references it.
- Add missing DELETE /sessions/:id/history endpoint to sessions.ts.
  The frontend session.js calls this endpoint for the "Clear history"
  button but no matching route existed.

Fixes #67

artifacts/api-server/src/routes/sessions.ts

@@ -569,4 +569,32 @@ router.post("/sessions/:id/topup", async (req: Request, res: Response) => {
   }
 });
 
+// ── DELETE /sessions/:id/history ─────────────────────────────────────────────
+
+router.delete("/sessions/:id/history", async (req: Request, res: Response) => {
+  const id = req.params.id as string;
+  const macaroon = extractMacaroon(req);
+
+  try {
+    const session = await getSessionById(id);
+    if (!session) { res.status(404).json({ error: "Session not found" }); return; }
+
+    if (!macaroon || macaroon !== session.macaroon) {
+      res.status(401).json({ error: "Invalid or missing macaroon" });
+      return;
+    }
+
+    if (session.state !== "active" && session.state !== "paused") {
+      res.status(409).json({ error: `Cannot clear history for a session in state '${session.state}'` });
+      return;
+    }
+
+    await db.delete(sessionMessages).where(eq(sessionMessages.sessionId, id));
+
+    res.json({ ok: true, sessionId: id });
+  } catch (err) {
+    res.status(500).json({ error: err instanceof Error ? err.message : "Failed to clear history" });
+  }
+});
+
 export default router;