fix: bundle pandoc+ffmpeg, fix CI pipeline and Windows GitHub build

- Remove package-lock.json from .gitignore so npm ci works in CI
- Refactor main.js: delegate PDF ops to src/main/PDFOperations.js,
  git ops to src/main/GitOperations.js
- getPandocPath(): use bundled binary from resources/bin/ when packaged,
  fall back to dev bin/ or system pandoc in development
- getFFmpegPath(): use ffmpeg-static (asarUnpack) when packaged
- Install ffmpeg-static (v5.3.0, bundled 76MB binary)
- Add scripts/download-tools.js to fetch pandoc binary at build time
  (idempotent, runs on CI before electron-builder)
- electron-builder: add asarUnpack for ffmpeg-static, extraFiles for
  pandoc binary per platform (linux + win32)
- release.yml: switch build-windows to windows-latest runner with native
  NSIS support; add cert decode step; add download-tools step for both
  linux and windows jobs
- Fix lint error: hoist outlinePanelContainer to module scope so
  TabManager methods can reference it without no-undef errors

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer.js

@@ -130,6 +130,9 @@ function plantumlEncode(text) {
     return '~h' + hex;
 }
 
+// Module-level reference set by DOMContentLoaded after sidebar registration
+let outlinePanelContainer = null;
+
 // Tab Management
 class TabManager {
     constructor() {