feat(composer): offer to format pasted code as a code block (#107)

When a multi-line paste looks like code, a chip above the composer asks
"That looks like code. Format it as a code block (js)?" with Format as code /
No thanks. It never converts on its own; typing, No, or 8 s dismiss it, and
Settings → Editor → "Offer to Format Pasted Code" turns it off.

- utils/looksLikeCode.ts (pure, tested): ≥ 3 lines and ≥ 2 of indentation
  with depth changes, statement terminators, operators + brackets, keywords,
  monospace/<pre> clipboard HTML, or SQL clause lines. Prose guards: quoted
  replies, URL lists, long sentence-punctuated lines, plain-word lines,
  markdown lists. Language guess only when fairly sure (js/ts/python/sql/
  rust/c/php).
- Accept rebuilds the pasted paragraphs as one code block from their plain
  text (code lines hold text only) and leaves the caret after it.
- Code blocks carry an optional lang → <code class="language-js"> (whitelisted
  identifier only).

Verified in Chromium: pasting a JS function shows the chip; Format as code →
sent formatted_body is <pre><code class="language-js"> with indentation intact;
a three-line prose paste shows no chip; typing after a paste dismisses it.
Unit tests: 14 detector fixtures + 2 output tests; chromium e2e 19 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/editor/output.ts

@@ -86,7 +86,10 @@ const elementToCustomHtml = (node: CustomElement, children: string): string => {
     case BlockType.CodeLine:
       return `${children}\n`;
     case BlockType.CodeBlock:
-      return `<pre><code>${children}</code></pre>`;
+      // [Gitea #107] Only a plain identifier is ever put in the class.
+      return node.lang && /^[a-z0-9+#-]{1,20}$/.test(node.lang)
+        ? `<pre><code class="language-${node.lang}">${children}</code></pre>`
+        : `<pre><code>${children}</code></pre>`;
     case BlockType.QuoteLine:
       return `${children}<br/>`;
     case BlockType.BlockQuote: