fix: resolve 12 critical runtime failures, security issue, and dead code

CRITICAL fixes:
- GitStatusPanel: use ipc.file.gitStage/gitCommit instead of non-existent top-level methods
- HeaderFooterDialog: use send/on channels instead of non-existent headerFooter namespace
- CodeMirrorEditor: use invoke('save-pasted-image') instead of non-existent savePastedImage
- ipc.ts: map to correct preload namespaces (git.status, git.stage, git.commit)
- Add pick-folder/pick-file to ALLOWED_SEND_CHANNELS whitelist
- Add git convenience namespace to preload (git.status/stage/commit/log/diff)

HIGH fixes:
- FindReplaceBar: replace broken match count stub with regex-based counter
- PDF export window: disable nodeIntegration, enable contextIsolation + sandbox
- Git handler: accept rootPath argument from renderer
- Add git-diff IPC handler + GitOperations.diff()

MEDIUM fixes:
- Remove 4 dead functions: checkPandocAvailable, safeExecFile, runPandoc, openPDFFile, exportWithPandocPDF
- Remove stale ODT header/footer console.log stub
- Rewrite electron.d.ts to match actual preload API shape

src/preload.js

@@ -105,6 +105,8 @@ const ALLOWED_SEND_CHANNELS = [
   'is-directory',
   'copy-path',
   'move-path',
+  'pick-folder',
+  'pick-file',
 
   // Git
   'git-status',
@@ -460,6 +462,15 @@ contextBridge.exposeInMainWorld('electronAPI', {
 
   getAppVersion: () => ipcRenderer.invoke('get-app-version'),
 
+  // Git Operations
+  git: {
+    status: (rootPath) => ipcRenderer.invoke('git-status', rootPath),
+    stage: (args) => ipcRenderer.invoke('git-stage', args),
+    commit: (args) => ipcRenderer.invoke('git-commit', args),
+    log: (rootPath) => ipcRenderer.invoke('git-log', rootPath),
+    diff: (filePath) => ipcRenderer.invoke('git-diff', filePath),
+  },
+
   app: {
     quit: () => ipcRenderer.send('app:quit'),
     openExternal: (url) => ipcRenderer.send('app:open-external', url),