What

Two usability bugs in the notes editor:

1. Clicking the image button in the toolbar did nothing useful (it inserted a bare `` placeholder, no file picker, no upload).
2. Clicking the preview (eye) icon felt like a one-way door — the toolbar greyed out and the way back wasn't obvious.

Why

1. Upload: EasyMDE has two toolbar items for images. `image` just inserts a markdown stub. `upload-image` is the one that fires the file-picker + `imageUploadFunction`. We had the former.
2. Preview: EasyMDE applies `disabled-for-preview` to other toolbar buttons while preview is active. Without explicit styling for that class + a clear title on the preview button, users had no signal that clicking the eye again would flip back.

Changes

`editor.js`

• Toolbar: `image` → `upload-image` (only when an upload URL is wired; falls back to `image` otherwise).
• Custom `preview` toolbar item with an explicit title: "Toggle preview (click again to go back)" and the `noDisable: true` flag so it stays fully interactive while siblings dim.
• Added `imageAccept` + `imageMaxSize` so paste / drag-drop match the server-side allowlist (png / jpg / webp / gif, 10 MB).
• Added `fullscreen` toolbar item for distraction-free writing.

`style.css`

• `.disabled-for-preview` and `button[disabled]` now render at 0.35 opacity — visibly greyed, not vanished — so the toolbar keeps its shape and the preview-exit button stays obvious.
• Active-state styling bumped from just-a-coloured-icon to a filled accent background so toggles read at a glance.

Verification

• File-picker button, drag-drop, and paste all route through the same endpoint (`POST /upload/note/{day|trade}/...`) → attachment row → markdown embed.
• Preview mode: eye icon stays fully coloured and clickable; other buttons dim to 0.35 opacity.
• Live round-trip:
  ```
  POST /upload/note/day/2026-04-15 → 200 {"url": "/media/2026/04/21/.png"}
  GET /media/2026/04/21/.png → 200 (image/png)
  ```

Checklist

• 46/46 tests green
• `uv run ruff check khata tests` clean
• `uv run ruff format khata tests` applied
• No schema change