fix(terminal): psmux wheel scrolling, per-platform tmux upgrade advice, Windows-safe server tests

- Server tests on Windows no longer reach the user's real psmux: psmux
  ignores TMUX_TMPDIR (and 3.3.x PSMUX_DATA_DIR), so quit.test.js's Quit and
  the runner's kill-server ended every live lane pane — the pre-commit hook
  included. scripts/test-server.js now takes tmux off the suites' PATH there.
- Wheel scrolling on Windows: psmux never turns mouse tracking on for its
  client, so xterm.js never reported the wheel and a pane could not scroll.
  A Windows attach is told {type:"wheel"} and the browser sends the SGR wheel
  report itself; psmux scrolls or forwards it to the pane's app.
- tmux.upgradeAdvice(): the oldest tmux AgentHub asks for and how each
  platform upgrades (psmux 3.4.0 -> winget upgrade marlocarlo.psmux; tmux 3.7
  -> brew upgrade tmux / build the release on Linux). Used by the doctor,
  GET /api/run/tmux ({available, version, upgrade}), `agenthub doctor`, and a
  sticky toast on the Workspace page. Advice only: nothing installs tmux.

Committed with --no-verify (user's call): the pre-commit hook runs the full
backend suite, which has ~378 pre-existing failures on Windows (S3/Linux-only
tests: sudo, /proc, systemd, members). Every test touching this change passes:
pty-attach, tmux, run, plugin-doctor-tmux, TerminalView, Workspace, and the
client tests related to the changed files; tsc and the header audit are clean.

client/src/i18n/locales/en/lanes.json

@@ -617,5 +617,6 @@
   "fx.readOnlyHome": "Another user's home — view and download only",
   "fx.readOnly": "Read-only",
   "fx.readOnlyHint": "This is another user's home: you can view, copy and download, not change it.",
-  "fx.homeEmpty": "Nothing in your home yet — its hidden files (.bashrc, .claude…) are not shown here. Your workspaces are on the left."
+  "fx.homeEmpty": "Nothing in your home yet — its hidden files (.bashrc, .claude…) are not shown here. Your workspaces are on the left.",
+  "tmuxUpgrade": "Upgrade tmux: {{advice}}"
 }

client/src/i18n/locales/vi/lanes.json

@@ -617,5 +617,6 @@
   "fx.readOnlyHome": "Home của user khác — chỉ xem và tải về",
   "fx.readOnly": "Chỉ đọc",
   "fx.readOnlyHint": "Đây là home của user khác: bạn chỉ xem, sao chép và tải về được, không sửa được.",
-  "fx.homeEmpty": "Home của bạn chưa có thư mục nào — các file ẩn (.bashrc, .claude…) không hiện ở đây. Các workspace nằm ở danh sách bên trái."
+  "fx.homeEmpty": "Home của bạn chưa có thư mục nào — các file ẩn (.bashrc, .claude…) không hiện ở đây. Các workspace nằm ở danh sách bên trái.",
+  "tmuxUpgrade": "Cần nâng cấp tmux: {{advice}}"
 }