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/main/GitOperations.js

@@ -0,0 +1,44 @@
+const simpleGit = require('simple-git');
+
+function getGitInstance(dir) {
+  return simpleGit(dir);
+}
+
+async function getStatus(dir) {
+  try {
+    const git = getGitInstance(dir);
+    return await git.status();
+  } catch (err) {
+    return { error: 'Not a git repository' };
+  }
+}
+
+async function stage(dir, files) {
+  try {
+    const git = getGitInstance(dir);
+    await git.add(files);
+    return await git.status();
+  } catch (err) {
+    return { error: err.message };
+  }
+}
+
+async function commit(dir, message) {
+  try {
+    const git = getGitInstance(dir);
+    return await git.commit(message);
+  } catch (err) {
+    return { error: err.message };
+  }
+}
+
+async function log(dir, maxCount = 20) {
+  try {
+    const git = getGitInstance(dir);
+    return await git.log({ maxCount });
+  } catch (err) {
+    return { error: err.message };
+  }
+}
+
+module.exports = { getStatus, stage, commit, log };