feat(M6): broadcast input + main-process ZMODEM + quick input panel + shortcuts

- broadcast: session target picker popover, fan-out write path, <2 targets auto-disable, per-tab target dots
- zmodem: main-process engine over ssh sessions (rz send / sz receive), progress reuses transfer panel, offer 120s / stall 90s watchdogs, e2e test with real zmodem.js peer (both directions)
- quick input panel: bottom-right overlay (command line + auto-\r + library + recent), broadcast-aware send
- shortcuts: Ctrl+=/-/0 font size (persisted), globalShowHide accelerator via globalShortcut, Ctrl+PgUp/PgDn terminal cycling
- contracts: ZMODEM_OFFER/RESPOND/DONE channels, SystemSettings.globalShowHide, TransferKind zmodem-upload/download
- review fixes: xterm-helper-textarea font hotkey pass-through, zmodem error-vs-cancelled progress state, offer timer clear, receive stream destroy

package.json

@@ -21,10 +21,12 @@
     "@xterm/xterm": "^6.0.0",
     "antd": "^6.6.2",
     "dockview-react": "^8.2.0",
+    "electron-updater": "^6.8.9",
     "font-list": "^2.1.0",
     "react": "^19.2.8",
     "react-dom": "^19.2.8",
     "ssh2": "^1.17.0",
+    "zmodem.js": "^0.1.10",
     "zustand": "^5.0.15"
   },
   "devDependencies": {
@@ -33,6 +35,7 @@
     "@types/react-dom": "^19.2.0",
     "@vitejs/plugin-react": "^5.0.0",
     "electron": "^44.2.0",
+    "electron-builder": "^26.15.3",
     "electron-vite": "^5.0.0",
     "typescript": "~5.9.0",
     "vite": "^7.0.0"