Website: add a Theming page documenting Styles, Glyphs, WithStyles, and SetStyles

[joestump-agent]

Part of #54. Best written once #56/#57 land so it documents the full token set.

Problem

The theming API is effectively undocumented outside the source:

• website/docs/api-reference.md shows Render(msgs, opts ...render.Option) but never says what an Option is; WithStyles, DefaultStyles, Styles, WithCompact, WithCompactThreshold appear nowhere on the website.
• The A2UI spec's own theming guide points users at each catalog's theming documentation ("check the documentation of each component to see what variables it exposes") — a2tea has no equivalent of the web catalog's CSS-variable reference.

Fix

Add a Theming page to the website (new slot in sidebars.ts under Guides):

• The layering story: agents send semantic hints; a2tea is the catalog; the host owns the final look via WithStyles — mirroring a2ui.org/guides/theming.
• The two invariants and why: monochrome default (no color SGR, test-pinned), createSurface.theme no-op (spec-aligned, upstream design open in Consider renaming theme on createSurface to configuration and remove primaryColor from basic catalog a2ui-project/a2ui#1118).
• Full reference table: every Styles slot and Glyphs entry, its default, and which components consume it — the terminal analog of the --a2ui-* variable reference.
• A worked example: mapping a host palette (primary/accent/subtle ramp) onto Styles, plus live restyle via SetStyles.
• Also document all render.Options in api-reference.md while there.