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

tests/commands-store.mjs

@@ -72,6 +72,7 @@ ok(store.listHistory().length === 0, 'clearHistory empties history')
 
 // ---- 4. Library CRUD + ordering ------------------------------------------------
 const a = store.saveLibraryItem({ command: 'ps aux', name: 'procs', note: 'show processes' })
+await wait(2) // createdAt has ms resolution; avoid a timestamp tie breaking the order assertion
 const b = store.saveLibraryItem({ command: 'df -h', name: 'disk', group: 'ops' })
 ok(typeof a.id === 'string' && a.id.length > 0, 'saveLibraryItem assigns an id + createdAt')
 let lib = store.listLibrary()