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>

.gitignore

@@ -16,7 +16,10 @@ out/
 .cache/
 .npm/
 .electron/
-package-lock.json
+# package-lock.json is intentionally tracked for reproducible CI builds
+
+# Downloaded tool binaries (fetched at build time via scripts/download-tools.js)
+bin/
 
 # Code signing certificates — never commit private keys
 *.pfx