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

package.json

@@ -37,6 +37,7 @@
     "@modelcontextprotocol/sdk": "^1.30.0",
     "@sveltejs/adapter-node": "^5.5.7",
     "@tiptap/core": "^3.31.0",
+    "@tiptap/extension-code-block-lowlight": "^3.31.0",
     "@tiptap/extension-link": "^3.31.0",
     "@tiptap/extension-placeholder": "^3.31.0",
     "@tiptap/extension-table": "^3.31.0",
@@ -54,6 +55,7 @@
     "better-sqlite3": "^11.10.0",
     "drizzle-orm": "^0.45.2",
     "idb": "^8.0.3",
+    "lowlight": "^3.3.0",
     "lucide-svelte": "^0.577.0",
     "markdown-it": "^15.0.1",
     "nanoid": "^5.1.16",