- Add FileCodeEditor.svelte: CodeMirror wrapper with auto language
detection, dark mode, Ctrl+S save, reactive to value/filePath changes
- Replace Shiki read-only highlighting + textarea editing with
always-editable CodeMirror for code files in FileNav preview
- Show persistent Save button for code files in toolbar
- Non-code text files keep existing Edit/Save/Cancel textarea flow
- SVG retains Shiki highlighting for visual preview mode
