[Bug] writeConfig is not atomic — mid-write crash corrupts user config

Problem

writeConfig calls writeFileSync directly on the target path. If the process is killed or the machine loses power mid-write, the config file is left in a partial state — subsequent readConfig calls trigger a JSON parse error and the user loses their entire configuration (see the companion silent-null bug in the same file).

Location

File: packages/desktop/src/main/workspace/config.ts:168-172

export function writeConfig(config: AppConfig): void {
  const cfgPath = configFilePath();
  const encrypted = encryptGatewayCredentials(config);
  writeFileSync(cfgPath, JSON.stringify(encrypted, null, 2), { encoding: 'utf-8', mode: 0o600 });
}

Fix Approach

Use the classic write-to-temp-then-rename pattern:

1. Write the encoded JSON to cfgPath + '.tmp' with the same mode: 0o600.
2. Call renameSync(cfgPath + '.tmp', cfgPath) — rename is atomic on both POSIX and NTFS.
3. On write failure, best-effort clean up the temp file before re-throwing the original error.

Verification

1. Run pnpm check — must pass.
2. Manual: force-kill the process immediately after a writeConfig invocation, relaunch — config must be either the old version or the new version, never a half-written mix.

Context

• WG: Artifact & File System
• Priority: Low
• Estimated effort: 20-30 minutes