Fix v1.9.0 logo upload and add MiKTeX PATH support

This commit fixes the logo upload issue and adds MiKTeX to the app's PATH for LaTeX/PDF support.
**Logo Upload Fix:**
- Replaced file input with Browse button that uses dialog.showOpenDialog
- Added browse-header-footer-logo IPC handler in main process
- Now gets reliable file paths without depending on file.path property
- Added browse-btn CSS styling
**MiKTeX PATH Addition:**
- Automatically adds MiKTeX bin directory to process.env.PATH on Windows
- Enables XeLaTeX/PDFLaTeX for PDF exports with headers/footers
- Logs PATH addition for debugging
**Files Changed:**
- src/main.js: Added MiKTeX PATH setup, browse logo IPC handler
- src/renderer.js: Changed to browseForLogo() function, updated event listeners
- src/index.html: Replaced file inputs with browse buttons
- src/styles.css: Added .browse-btn styling
**Testing Notes:**
- Logo upload now works without "path undefined" errors
- DOCX exports with headers/footers confirmed working
- PDF exports should now use Pandoc+XeLaTeX with MiKTeX in PATH
- Requires MiKTeX installation and PC restart for full functionality
🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/styles.css

@@ -3312,6 +3312,22 @@ body.printing-no-styles .preview-content pre,
     background: var(--button-hover, #5a6268);
 }
 
+.browse-btn {
+    padding: 6px 12px;
+    background: var(--accent-color, #007bff);
+    color: white;
+    border: none;
+    border-radius: 4px;
+    cursor: pointer;
+    font-size: 13px;
+    transition: background 0.2s ease;
+    margin-right: 8px;
+}
+
+.browse-btn:hover {
+    background: var(--accent-hover, #0056b3);
+}
+
 .clear-btn {
     padding: 6px 12px;
     background: var(--danger-color, #dc3545);