Make Notes UI better + add light theme

### Project slug (e.g., snake, todo)

notes

### Summary

The Notes UI needs a visual overhaul and a light theme. Improvements: clearer note list, improved editor layout (title, tags, body), autosave indicator, theme-aware editor colors, and better mobile UX.

### Details / Acceptance criteria

Light theme available and applied to Notes page/editor.

Note editor supports clear title, tag metadata, and large editing area with proper padding/line-height.

Autosave status indicator visible; undo/redo supported if feasible.

Notes list shows preview, last-edited timestamp, and supports search/sort.

Mobile-friendly layout for reading and editing.

### Scope

- [x] UI changes
- [x] Logic changes
- [x] Accessibility
- [x] Tests (manual)