Draw Mermaid labels as HTML so arrows and entities read as written (#312)

Mermaid's SVG-text label path mangles common planning text: on 11.12.0,
`Vec<T>`, `#quot;` and `#gt;` showed up as `&lt;T&gt;`, `&quot;` and `&gt;`,
and on 11.17 a bare `>` in `->` or `=>` was dropped. That path only runs
because Chopin turned HTML labels off.

Turn HTML labels back on, as GitHub, GitLab, Docusaurus and Obsidian
do. securityLevel stays "strict", so labels are still sanitized with
DOMPurify and scripts and click handlers stay off. Upgrade Mermaid to
11.17.2. The Chopin theme already styles HTML node and edge labels.

Fixes #312

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-duration: 15m converged, estimated unmeasured
Assistant-verification: playwright e2e passed: new "diagram labels keep arrows, operators and entity codes as written" fails on main's config (labels read `Vec&lt;T&gt;`, `say &quot;hi&quot; ... &gt; 0`) and passes with this change
Assistant-verification: playwright e2e passed: bun run e2e e2e/code.e2e.ts e2e/responsive-content.e2e.ts, 28 passed
Assistant-verification: bun test passed: 3669 tests, 3 skip, 0 fail
Assistant-verification: bun run types passed: all workspaces
Assistant-verification: bun run ci passed: dprint, oxlint, tokens, design contract, design record, Impeccable (no new findings)
Assistant-verification: manual browser comparison passed: flowchart, sequence, class, state and ER screenshots before and after keep the Chopin theme; HTML labels add slightly taller line spacing in nodes
User-preference: Prefer what popular projects do (Mermaid HTML labels under strict) over custom patches to dependencies
Co-authored-by: Alex Lavaee <lavaman131@github.com>