fix(input): durable exactly-once input delivery so a dropped link can't lose a prompt

A "sent" prompt could vanish with no trace on a flaky connection (e.g. a train):
with local echo on, Enter cleared the overlay then sent over the WebSocket
fire-and-forget. On a half-open socket (readyState===OPEN, dead TCP) ws.send()
doesn't throw, so the frame was silently discarded, nothing was enqueued, and
navigator.onLine stayed true — the prompt was lost and never resent.

Replace the best-effort offline queue with a durable, acknowledged delivery layer:

- Client (app.js): every input frame is recorded with a stable clientId +
  monotonic per-session seq and persisted to localStorage BEFORE delivery, and
  only dropped on a server ACK. Delivered over WS (acked via {t:'ia',seq}) or,
  when the socket is down, POST in seq order (HTTP 2xx = ACK). A 2s sweep
  force-reconnects a WS whose oldest frame is unacked past 4s (half-open sockets
  never recover on their own); on reconnect/reload all pending frames re-deliver.
  Survives reconnects AND page reloads. Connection indicator shows pending count.
- Server: Session.shouldApplyInput(clientId, seq) applies each frame exactly once
  (bounded MRU map); ws-routes + POST /input dedup a redelivered seq but still ACK
  it (200 / {t:'ia'}), so an at-least-once resend can never type the prompt twice.
  Untagged input (curl/legacy) applies unconditionally — no behavior change.
- terminal-ui.js sendInput() (voice / keyboard-accessory / paste) now routes
  through the same durable layer.

Tests: test/reliable-input-dedup.test.ts (exactly-once semantics on the real
Session) + POST /input dedup route tests. Design: docs/reliable-input-delivery.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/routes/session-routes.test.ts

@@ -355,6 +355,43 @@ describe('session-routes', () => {
       const body = JSON.parse(res.body);
       expect(body.success).toBe(false);
     });
+
+    it('applies a tagged (clientId, seq) input exactly once on redelivery', async () => {
+      const url = `/api/sessions/${harness.ctx._sessionId}/input`;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const session = harness.ctx.sessions.get(harness.ctx._sessionId) as any;
+      session.writeBuffer.length = 0;
+
+      const post = (payload: unknown) => harness.app.inject({ method: 'POST', url, payload });
+
+      // First delivery of seq 1 — applied (200, written once).
+      const first = await post({ input: 'prompt', seq: 1, clientId: 'cid-1' });
+      expect(first.statusCode).toBe(200);
+
+      // Redelivery of the SAME seq (client never saw the ACK) — still 200, but
+      // must NOT write again.
+      const dup = await post({ input: 'prompt', seq: 1, clientId: 'cid-1' });
+      expect(dup.statusCode).toBe(200);
+
+      // A genuinely new seq — applied.
+      const next = await post({ input: '\r', seq: 2, clientId: 'cid-1' });
+      expect(next.statusCode).toBe(200);
+
+      expect(session.writeBuffer).toEqual(['prompt', '\r']);
+    });
+
+    it('always applies untagged input (curl/legacy, no dedup)', async () => {
+      const url = `/api/sessions/${harness.ctx._sessionId}/input`;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const session = harness.ctx.sessions.get(harness.ctx._sessionId) as any;
+      session.writeBuffer.length = 0;
+
+      const post = () => harness.app.inject({ method: 'POST', url, payload: { input: 'x' } });
+      await post();
+      await post();
+      // No seq/clientId ⇒ no dedup ⇒ both writes land.
+      expect(session.writeBuffer).toEqual(['x', 'x']);
+    });
   });
 
   // ========== POST /api/sessions/:id/resize ==========