[Bugfix #442] Add terminal WebSocket auto-reconnect with session resumption

[waleedkadous]

Summary

Fixes #442

Root Cause

The dashboard Terminal component had zero reconnection logic — when a WebSocket closed (common on cloud connections), it displayed [Terminal disconnected] and stopped permanently. The only recovery was a full page reload.

Fix

Client-side (Terminal.tsx):

• Auto-reconnect with exponential backoff (1s, 2s, 4s, 8s, 16s, 30s cap)
• Track sequence numbers from server seq control frames
• On reconnect, pass ?resume=N query param for session resumption from ring buffer
• Show yellow "Reconnecting..." overlay during reconnection attempts
• Detect rapid failures (session gone, e.g. builder terminated) — stops after 5 rapid failures
• Give up after 15 total attempts with permanent disconnect message
• Reset counters on successful reconnection

Server-side (tower-websocket.ts):

• Send seq control frame with current ring buffer sequence after replay
• Periodic 10-second heartbeat with updated sequence number
• Support reading resume sequence from URL query param (browser WebSocket API doesn't support custom headers)
• Clean up heartbeat interval on connection close

Protocol (ws-protocol.ts):

• Added seq to the ControlMessage type union

Test Plan

• Added Terminal.reconnect.test.tsx (10 tests): backoff timing, seq tracking, overlay show/hide, rapid failure detection, max attempts, unmount cleanup
• Updated tower-websocket.test.ts (+4 tests): seq frame after replay, heartbeat, query param resume, interval cleanup
• All 48 directly related tests pass (tower-websocket: 25, ring-buffer: 15, ws-protocol: 8)
• TypeScript compiles cleanly (both packages/codev and dashboard)
• No regressions in existing tests (7 pre-existing failures on main confirmed)

CMAP Review

To be added after review