fix(main): SFTP data integrity, session lifecycle, security hardening

- upload: per-chunk buffer (ssh2 re-reads the overflow tail after the ACK;
  a reused buffer silently corrupted every file >= ~254KB)
- close the cached SFTP channel on eviction, attach an 'error' handler,
  close the download handle, time out execQuiet, fail partial deletes
- per-session StringDecoder for the ssh data plane (CJK mojibake), real
  exit codes, safe replay truncation, zmodem abort/counter/timer fixes
- sysinfo: idempotent poll end, error routing, per-poll watchdog, proc(5)
  CPU total; expand cd ~/$HOME/%USERPROFILE% paths; log sanitizer fixes
- security: will-navigate guard, central IPC sender check, scheme
  allowlist for openExternal, single-instance else branch, layout id and
  log-name whitelists, custom theme sanitizing, atomic JSON writes with
  EPERM retry in store.writeJson
- updater: per-attempt feed choice, quitAndInstall relaunch, dev guard,
  update-state getter

src/main/knownHosts.ts

@@ -5,8 +5,8 @@
  */
 
 import { createHash, randomUUID } from 'crypto'
-import { mkdirSync, readFileSync, writeFileSync } from 'fs'
-import { dirname } from 'path'
+import { readFileSync } from 'fs'
+import { writeJson } from './store'
 
 export interface KnownHostEntry {
   /** uuid; addedAt is split out so fingerprint clash updates can be precise */
@@ -63,8 +63,7 @@ export class KnownHostsStore {
   }
 
   private save(shape: KnownHostsStoreShape): void {
-    mkdirSync(dirname(this.filePath), { recursive: true })
-    writeFileSync(this.filePath, JSON.stringify(shape, null, 2), 'utf8')
+    writeJson(this.filePath, shape)
   }
 
   /**