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
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