feat(server): bake a snippet kit into every snippet doc

[benvinegar]

What

Bakes a "snippet kit" into every snippet document so agents publish compact markup instead of hand-writing inline CSS for every button, slider, and diagram label — fewer output tokens means faster publishes, and snippets get a consistent look for free.

• Element defaults: bare button, input, select, textarea are pre-styled to match the viewer (hover/focus included); checkboxes, radios, ranges, and progress bars are themed via accent-color. color-scheme: light dark makes native controls render dark automatically.
• SVG utility classes: t/ts/th text presets, box (neutral rect, rx: 8 via CSS), arr (1.2px connector), leader (dashed guide), node (cursor + hover dim), and c-blue|teal|amber|coral|green|red|gray color ramps that set fill+stroke on shapes (or a whole <g>, including descendant .box rects) and auto-switch child <text> to readable ink in light and dark. Blue/amber/green/red/gray alias the existing semantic tokens; teal and coral get their own light/dark variable pairs.
• Shared arrow marker: a <marker id="arrow"> is injected into every doc; fill="context-stroke" makes the arrowhead inherit each line's stroke, so one marker replaces per-color marker defs.
• The design guide gains a "Built-in kit" reference table (+ Tabler icons one-liner) and the theming prose is trimmed to "for anything the kit doesn't cover"; the skill doc points at the kit first.
• Validation: the demo JWT diagram rewritten with the kit drops from 2,893 to 1,555 bytes (−46%) and renders identically in light and dark.

Reviewer notes

• CSS rules override SVG presentation attributes, so the kit deliberately never styles bare svg text (fill/font-size) — that would silently break existing snippets that use fill="..." or font-size="..." attributes. There's a comment in snippetPage.ts pinning this constraint.
• context-stroke needs Chrome 119+ / Safari 18+; older browsers would render a black arrowhead.
• Verified in the browser on the demo content: light + dark screenshots of the JWT diagram and the backoff explainer's form controls. npm test (49), both typecheck programs, lint, format, and e2e (10/10, chromium + webkit) all pass.

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