feat(editor): make code snippets readable and syntax highlighted

Closes #90.

Code blocks and inline code inherited the typography plugin's defaults,
which shrank them to ~86% of an already-monospace body and painted them
--bg-surface — the same colour as the default note card — so a snippet
was only recognisable by being smaller. Long lines also grew a horizontal
scrollbar inside card previews.

- New --code-bg surface (one step below the page in light mode, page-level
  ink in dark) with a subtle border, and code keeps the surrounding text
  size. Decorative backtick pseudo-elements dropped now that the chip
  carries the signal.
- Card previews clip wide code instead of scrolling; editor and share page
  keep horizontal scroll.
- Syntax highlighting via one shared lowlight instance feeding both the
  TipTap editor (extension-code-block-lowlight) and the markdown-it
  renderer, so a fence looks identical in the editor, on cards and on the
  share page. Only highlight.js "common" grammars are bundled.
- No auto-detection: highlight.js misreads note-sized snippets too often
  (JavaScript as CSS, SQL as VB.NET). Unlabelled fences stay plain; the
  language comes from the fence info string or a toolbar picker shown
  while the cursor is in a code block.
- Six --hl-* token colours per theme; a unit test parses app.css and
  asserts AA contrast against --code-bg and mutual distinguishability.
- Sanitizer now allows only language-* / hljs-* classes on code and span.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014u5WimmZvshm9RJSd1c6wx

CLAUDE.md

@@ -39,6 +39,8 @@ All colors are CSS variables in `src/app.css`. Never use hardcoded Tailwind colo
 | `--card-shadow-hover` | `3px 3px 0px` | Hover shadow in primary gold |
 | `--accent-check` | `var(--primary)` (light) / `#c8a44a` (dark) | Checkbox accent color — dimmed gold in dark mode so a long done-list doesn't shout |
 | `--hover-wash` | `var(--border)` (light) / `#ece3d3` (dark) | Tint source for hover/active washes (`bg-[var(--hover-wash)]/N`) — kept cream in dark mode since `--border`'s mid-grey is invisible at low opacity |
+| `--code-bg` | `#dfd5c1` (light) / `#141312` (dark) | Code block and inline code surface — one step below the page so it stands out on every note colour, including the default card which is `--bg-surface` itself |
+| `--hl-keyword` … `--hl-tag` | six token colours per theme | Syntax highlighting roles (keyword, string, number, comment, title, tag). Must keep AA contrast against `--code-bg`; asserted in `src/lib/utils/highlight.test.ts` |
 
 Note colors for cards are defined in `src/lib/utils/colors.ts`.