Add optional Electron desktop packaging with bundled mini-bash

[yeyan00]

Summary

This adds an optional Electron desktop wrapper for Pi Web while leaving the existing web/CLI runtime unchanged.

Closes #201.

What changed

• Added electron/main.cjs desktop launcher that:
  • starts the existing Next.js server locally,
  • opens it in an Electron BrowserWindow,
  • defaults PI_CODING_AGENT_DIR to Electron userData/agent,
  • supports --agent-dir <path> and --portable,
  • adds bundled Windows mini-bash to PATH when present,
  • initializes desktop settings.json.shellPath to bundled bash.exe when no valid shell path exists.
• Added desktop packaging scripts:
  • desktop:dev
  • desktop:pack
  • desktop:dist
• Added electron-builder config for a Windows x64 portable exe.
• Added vendor/mini-bash for Windows bash/coreutils support.
• Documented desktop packaging and runtime behavior in English and Chinese READMEs.

Validation

• node_modules/.bin/tsc --noEmit
• npm run desktop:dist
• Smoke-tested unpacked app with:
  • dist-desktop/win-unpacked/Pi Web.exe --portable --port 30142
  • confirmed http://127.0.0.1:30142/ returns 200 OK
  • confirmed portable data dir and settings.json.shellPath point at bundled mini-bash

Notes

• The desktop wrapper is optional and does not change the existing npm CLI/browser flow.
• Bundled mini-bash covers common bash/coreutils commands, but does not include Git, Node, npm, or npx.
• Git/worktree functionality still requires Git on PATH.
• Skill install/search may still require Node/npm/npx.
• The committed mini-bash binaries may require additional license/third-party notice review before merging/release.