feat(import): one Import-file entry + vault notebooks

Phase 3. Import becomes a single top-level action beside "Create
notebook" and the library is vault-backed.

- VaultService.createNotebook copies a picked file into a fresh
  (de-duplicated) notebook folder under the vault; its sidecar lives
  beside it, so annotations travel with the file.
- Home screen: one "Import file" action with a multi-extension picker
  (pdf / docx / pptx); routes to the editor by extension.
- The document list is now a vault scan (folders with a source file),
  not the SQLite documents table — no cache, always correct.
- PPTX soffice detection fix; DOCX convert-on-import is best-effort
  and fails gracefully when LibreOffice is unavailable.

analyze clean, tests green.

lib/services/pptx_service.dart

@@ -127,13 +127,15 @@ class PptxService {
   /// Try converting via LibreOffice headless.
   Future<List<String>> _convertViaLibreOffice(String pptxPath) async {
     try {
-      // Check if LibreOffice is available
-      final which = await Process.run('which', ['libreoffice']);
-      if (which.exitCode != 0) return [];
+      // Resolve the LibreOffice binary across platforms. On Windows the binary
+      // is `soffice.exe` (not on PATH for `which`, which is POSIX-only), so we
+      // probe the standard install locations as well — see resolveSoffice().
+      final soffice = await resolveSoffice();
+      if (soffice == null) return [];
 
       final outDir = await _makeTmpDir('pptx_images');
 
-      final result = await Process.run('libreoffice', [
+      final result = await Process.run(soffice, [
         '--headless',
         '--convert-to',
         'png',
@@ -174,6 +176,73 @@ class PptxService {
     }
   }
 
+  /// Resolve the LibreOffice CLI binary for the current platform, or null when
+  /// it cannot be found.
+  ///
+  /// Order:
+  ///   1. Windows: `soffice.exe` at the standard install paths
+  ///      (`C:\Program Files\LibreOffice\program\soffice.exe`, and the 32-bit
+  ///      `Program Files (x86)` variant). The POSIX `which` can't find these.
+  ///   2. POSIX: `which libreoffice`, then `which soffice` (macOS/some distros).
+  ///   3. Otherwise null → callers fall back gracefully.
+  static Future<String?> resolveSoffice() async {
+    if (Platform.isWindows) {
+      const candidates = [
+        r'C:\Program Files\LibreOffice\program\soffice.exe',
+        r'C:\Program Files (x86)\LibreOffice\program\soffice.exe',
+      ];
+      for (final c in candidates) {
+        if (await File(c).exists()) return c;
+      }
+      // Last resort: maybe soffice is on PATH (e.g. a portable install).
+      if (await _whichOk('soffice')) return 'soffice';
+      return null;
+    }
+    if (await _whichOk('libreoffice')) return 'libreoffice';
+    if (await _whichOk('soffice')) return 'soffice';
+    return null;
+  }
+
+  static Future<bool> _whichOk(String cmd) async {
+    try {
+      final r = await Process.run('which', [cmd]);
+      return r.exitCode == 0;
+    } catch (_) {
+      return false;
+    }
+  }
+
+  /// Convert an arbitrary office document (e.g. DOCX) to PDF via LibreOffice
+  /// headless, writing the PDF NEXT TO [sourcePath] (same folder, same
+  /// basename + `.pdf`). Returns the PDF path on success, or null when
+  /// LibreOffice is unavailable or the conversion fails — callers MUST handle
+  /// null and surface a friendly message rather than crash.
+  Future<String?> convertToPdf(String sourcePath) async {
+    final soffice = await resolveSoffice();
+    if (soffice == null) return null;
+
+    final outDir = p.dirname(sourcePath);
+    try {
+      final result = await Process.run(soffice, [
+        '--headless',
+        '--convert-to',
+        'pdf',
+        '--outdir',
+        outDir,
+        sourcePath,
+      ]);
+      if (result.exitCode != 0) return null;
+      final pdfPath = p.join(
+        outDir,
+        '${p.basenameWithoutExtension(sourcePath)}.pdf',
+      );
+      if (await File(pdfPath).exists()) return pdfPath;
+      return null;
+    } catch (_) {
+      return null;
+    }
+  }
+
   /// Generate placeholder slide images when LibreOffice is not available.
   ///
   /// Uses ImageMagick `convert` to create PNG files with slide numbers.