fix: improve auto-update UX and proxy support, bump v0.17.1

Auto-update flow rework:
- updater.ts: disable autoDownload, add manual `updater:download` IPC handler
  so user explicitly triggers download via "Download & Install" button
- updater.ts: resolve system proxy via session.defaultSession.resolveProxy()
  and inject into HTTPS_PROXY env var, fixing zero download speed behind VPN/proxy
- preload.ts + electron.d.ts: expose `downloadUpdate` method to renderer

UI improvements:
- UpdateDialog: add "Download & Install" button when update available but not
  yet downloading; show progress bar during download; "Restart to Update" when ready
- GeneralSection UpdateCard: show "Download & Install" / "Restart to Update" button
  next to "Check for Updates"; show download progress bar inline; add "View Details"
  link to reopen update dialog
- AppShell: dismissUpdate no longer permanently hides update for that version
  (removed localStorage persistence), just closes the dialog
- useUpdate: add downloadUpdate to context

i18n:
- Added `update.installUpdate` key (en: "Download & Install", zh: "下载并安装")

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "codepilot",
-  "version": "0.17.0",
+  "version": "0.17.1",
   "private": true,
   "author": {
     "name": "op7418",