feat(editor): resizable outline panel (#161)

Add a drag handle to resize the outline panel and persist its width in
vault state (outline_width). Outline text now scales with the editor
font size.

Reviewed-on: https://codeberg.org/ArkHost/HelixNotes/pulls/161

src/lib/types.ts

@@ -129,6 +129,7 @@ export interface VaultState {
   last_open_note: string | null;
   sidebar_width: number;
   notelist_width: number;
+  outline_width?: number;
   sidebar_collapsed: boolean;
   notelist_collapsed: boolean;
   collapsed_notebooks: string[];