feat: responsive web terminal with auto-reconnect, heartbeat, and local echo

Implements a resilient, responsive web terminal that survives network blips,
provides instant typing feedback, and restores scrollback on reconnect.

Backend changes:
- Add heartbeat tracking (15s ping interval, 60s idle timeout)
- Add message batching (16ms flush window) for efficient I/O
- Add termios echo detection and set_echo_state control messages
- Add graceful session_ended notification before close
- Add ping/pong protocol support

Frontend changes:
- Rewrite TerminalComponent with status bar, connection indicator,
  session-ended overlay, reconnect banner, and ResizeObserver
- Add useTerminalConnection hook with:
  - Exponential backoff auto-reconnect (1s → 30s max, 10 attempts)
  - Heartbeat/ping-pong with latency tracking
  - Local echo for printable ASCII with server deduplication
  - Resize debounce (200ms) + throttle (500ms)
  - Scrollback serialization via xterm-addon-serialize
  - Ctrl+Shift+R manual reconnect shortcut
- Add WebSocket protocol types and encoding utilities
- Add xterm-addon-serialize dependency

Tests:
- 16 backend unit tests (TerminalSession + TerminalManager)
- 13 frontend hook tests (connection lifecycle, reconnect, resize,
  scrollback, callbacks)

Quality gates:
- Frontend typecheck: clean
- Frontend lint: clean
- Frontend tests: 48 passed
- Backend unit tests: 101 passed
- Backend ruff: clean

SDD artifacts: openspec/changes/responsive-terminal/

apps/web/package-lock.json

@@ -19,6 +19,7 @@
         "tailwindcss": "^3.3.0",
         "xterm": "^5.3.0",
         "xterm-addon-fit": "^0.8.0",
+        "xterm-addon-serialize": "^0.11.0",
         "xterm-addon-web-links": "^0.9.0"
       },
       "devDependencies": {
@@ -6372,6 +6373,16 @@
         "xterm": "^5.0.0"
       }
     },
+    "node_modules/xterm-addon-serialize": {
+      "version": "0.11.0",
+      "resolved": "https://registry.npmjs.org/xterm-addon-serialize/-/xterm-addon-serialize-0.11.0.tgz",
+      "integrity": "sha512-2CNDnmLdLkNWfsxNFkGsI5FE9W/BbsMzeOrbu59yNqH9L6k1gmL+Ab6VXxEp2NQUJSzaiqi6t0nFR5k5EDkVIg==",
+      "deprecated": "This package is now deprecated. Move to @xterm/addon-serialize instead.",
+      "license": "MIT",
+      "peerDependencies": {
+        "xterm": "^5.0.0"
+      }
+    },
     "node_modules/xterm-addon-web-links": {
       "version": "0.9.0",
       "resolved": "https://registry.npmjs.org/xterm-addon-web-links/-/xterm-addon-web-links-0.9.0.tgz",