Storybook: cover every enabled markdown feature in MarkdownPreview stories

[Alimedhat000]

Context

PR #60 (#49) introduced behavior specs for the markdown preview as Storybook browser tests (markdown-preview.stories.tsx): XSS sanitization, mermaid rendering (incl. label text), and a small safe-content sample (hljs, links, GFM task lists, KaTeX). That covers the security surface, but the preview enables many more features that have zero automated coverage — any pipeline/schema change could silently break them.

Enabled today (see MarkdownPreview.tsx + docs/client/markdown-preview-sanitization.md): remarkTypographer, remarkGfm, remarkMath, remarkEmoji, rehypeTextDecorations, rehypeSupSub, rehypeKatex, rehypeHighlight, rehypeSlug, mermaid (decoupled renderer).

Scope

One story per feature family (or a matrix story), each asserting the feature renders and survives sanitization:

• GFM: tables, strikethrough, autolinks, footnotes (+ footnote backrefs), nested lists, blockquotes
• Task lists (checked + unchecked states)
• Typographer: smart quotes, dashes, ellipses
• Emoji shortcodes
• Inline math + display math ($$...$$) via KaTeX
• Subscript/superscript (rehypeSupSub syntax)
• Text decorations (rehypeTextDecorations — underline/marks/etc.)
• Code fences in several languages → hljs + language-* classes present
• Heading ids from rehypeSlug + MarkdownToc entries matching
• Images (http[s] src allowed by schema) + relative-link handling
• Mermaid: sequence, class, state, gantt, pie, mindmap — each renders svg with visible label text (flowchart already covered; other diagram families use different label paths — this is where htmlLabels/foreignObject regressions would hide)

Acceptance criteria

• All stories green under pnpm exec vitest run --project storybook
• Feature docs updated (docs/client/markdown-preview-sanitization.md testing section)
• No story relies on implementation internals; assert rendered DOM behavior

Related: #49, PR #60