test(main): cover updater fallback, ipc sender guard, log sanitizer, sftp timeouts

- updater-fallback.mjs (82 assertions): GitHub probe fallback to Gitea,
  timeout budgets, in-flight check never stuck in 'checking'; updater.ts
  gains a setUpdateTimeouts test seam, electron-updater aliased to a stub
- ipc-guard.mjs (43): trusted-frame guard exercised through real
  registerIpc handlers with forged senderFrames; electron-stub now records
  registrations via globalThis so bundle and test share one instance
- log-sanitizer.mjs (71): CSI/OSC/charset state machine, alt-screen fold,
  byte-split fuzz equal to whole-chunk output; fixes a wrong comment
- sftp-timeout.mjs (39): per-op timeouts (metadata 30s, transfer chunk 60s,
  open 10s) evict half-dead channels with one retry, slow-but-progressing
  transfers untouched, late rejections never unhandled
- reservedAccelerators.ts: single pure isReservedAccelerator shared by the
  settings recorder and applyGlobalShortcut (the two tables had drifted —
  main now also refuses Ctrl+=/-/0/PgUp/PgDn legacy values); 56 assertions
- ssh-loopback.mjs loads the real ssh.ts via a bundle (50 assertions):
  TOFU pinning, fail-closed stores, auth gate, connect budget

Offline suite grows 13 -> 17. Renderer test framework evaluated: not
introducing vitest/jsdom; pure logic keeps being extracted and tested
through the existing bundle harness.

tests/electron-stub.cjs

@@ -2,24 +2,76 @@
 // (tests/ssh-session-e2e.mjs, tests/commands-store.mjs). Only what the bundled
 // modules touch.
 //
-// `app.getPath('userData')` is configurable via `__setUserData` so a test can
-// point the settings store at a temp dir and exercise settings-driven behavior.
-let userDataPath = process.env.OT_STUB_USERDATA || ''
+// Every mutable piece of state lives on `globalThis.__otElectronStub` rather
+// than in this module's scope: the bundles INLINE this file (esbuild aliases
+// `electron` to it), so a test requiring both `./electron-stub.cjs` and a
+// bundle would otherwise get two independent copies, and registrations made by
+// the bundled code would be invisible to the test.
+//
+// `app.getPath('userData')` defaults to `OT_STUB_USERDATA` and is also settable
+// via `__setUserData`, so a test can point the settings store at a temp dir and
+// exercise settings-driven behavior.
+const state = (globalThis.__otElectronStub ??= {
+  handlers: new Map(),
+  userDataPath: process.env.OT_STUB_USERDATA || '',
+  isPackaged: false,
+  sessions: new Map()
+})
+
+/** Fetch double for the updater bundle: `net.fetch` and every session's fetch. */
+const noFetch = async () => ({
+  ok: false,
+  status: 404,
+  text: async () => '',
+  json: async () => []
+})
+const callFetch = (url, init) =>
+  globalThis.__otFetch ? globalThis.__otFetch(url, init) : noFetch()
+
+const fakeSession = (name) => {
+  let s = state.sessions.get(name)
+  if (!s) {
+    s = {
+      name,
+      proxyCalls: [],
+      setProxy: async (cfg) => {
+        s.proxyCalls.push(cfg)
+      },
+      fetch: callFetch
+    }
+    state.sessions.set(name, s)
+  }
+  return s
+}
+
 module.exports = {
   BrowserWindow: {
     getAllWindows: () => []
   },
   app: {
     getPath: (name) => {
-      if (name === 'userData' && userDataPath) return userDataPath
+      if (name === 'userData' && state.userDataPath) return state.userDataPath
       throw new Error(`electron stub: app.getPath(${name}) is not configured`)
     },
+    getVersion: () => '0.0.0-stub',
     setLoginItemSettings: () => {},
-    isPackaged: false
+    get isPackaged() {
+      return state.isPackaged
+    }
   },
   ipcMain: {
-    handle: () => {},
-    on: () => {}
+    handle: (channel, listener) => {
+      state.handlers.set(channel, listener)
+    },
+    on: (channel, listener) => {
+      state.handlers.set(`on:${channel}`, listener)
+    }
+  },
+  session: {
+    fromPartition: (name) => fakeSession(name)
+  },
+  net: {
+    fetch: callFetch
   },
   globalShortcut: {
     register: () => true,
@@ -39,6 +91,9 @@ module.exports = {
   shell: {
     openPath: async () => ''
   },
+  dialog: {
+    showOpenDialog: async () => ({ canceled: true, filePaths: [] })
+  },
   safeStorage: {
     isEncryptionAvailable: () => false
   },
@@ -63,6 +118,16 @@ module.exports = {
     createFromPath: () => ({ isEmpty: () => true, resize: () => ({}) })
   },
   __setUserData: (p) => {
-    userDataPath = p
+    state.userDataPath = p
+  },
+  __setPackaged: (v) => {
+    state.isPackaged = v
+  },
+  /** channel -> listener (invoke), `on:<channel>` -> listener (send). */
+  get __handlers() {
+    return state.handlers
+  },
+  get __sessions() {
+    return state.sessions
   }
 }