refactor: unified EditorPanel for notes and files, SaveFileText backend

- Create EditorPanel.svelte: shared editor component for notes + files
  - Markdown: edit/preview/split modes via MarkdownEditor + MarkdownPreview
  - Plain text: monospace textarea
  - Header: title, mode switcher, external open button
  - Footer: save + close buttons, dirty indicator
- Rewrite NoteEditorPanel.svelte as thin wrapper over EditorPanel
- FilesTab: browser/editor modes (editor replaces file list, not appended)
- Add SaveFileText Wails binding: writes text files atomically (tmp+rename)
- Add WriteText to files.Service with SHA256 update
- Remove separate TextFileEditor.svelte (replaced by EditorPanel)
- Note context: SaveNote API, noteId/title props
- File context: SaveFileText API, fileId prop
- Linked .md (CheckFileAction=note) → note editor tab
- Unlinked .md / text files → EditorPanel in FilesTab
- Images/PDF/binary → FilePreviewModal only

Co-Authored-By: OWL (Hermes Agent) <hermes@nousresearch.com>

internal/core/files/file.go

@@ -330,6 +330,28 @@ func (s *Service) ReadText(id string) (string, error) {
 	return string(b), nil
 }
 
+// WriteText writes text content to a file on disk and updates the record.
+func (s *Service) WriteText(rec *Record, content string) error {
+	abs, err := s.absPathSafe(rec)
+	if err != nil {
+		return err
+	}
+	tmp := abs + ".tmp"
+	if err := os.WriteFile(tmp, []byte(content), 0o640); err != nil {
+		return fmt.Errorf("write temp: %w", err)
+	}
+	if err := os.Rename(tmp, abs); err != nil {
+		os.Remove(tmp)
+		return fmt.Errorf("rename: %w", err)
+	}
+	now := time.Now().UTC().Format(time.RFC3339)
+	sum := sha256.Sum256([]byte(content))
+	_, err = s.db.Exec(
+		`UPDATE files SET size=?, sha256=?, updated_at=?, missing=? WHERE id=?`,
+		len(content), fmt.Sprintf("%x", sum[:]), now, 0, rec.ID)
+	return err
+}
+
 // ReadBase64 reads a file and returns a data URI (base64-encoded).
 func (s *Service) ReadBase64(id string) (string, error) {
 	rec, err := s.Get(id)