feat(config): brand_href + dark/light code themes — the knobs API sites shim today

[mhenrixon]

Summary

Three config knobs replace the workarounds sites (zazu/app-4) shim by subclassing DocsUI::Shell today — the app-4 Shell subclass shrinks to nothing once these land.

Knob	Default	What it does
c.brand_href	"/"	The topbar brand link target. Shell#topbar reads it, so pointing the brand at /docs is one line — not a copy-pasted #topbar subclass (flagged fragile in app-4's own comments).
c.code_theme_dark	nil	A second Rouge theme for dark daisyUI themes. nil keeps the single-theme behavior byte-for-byte (backwards compatible). When set, DocsUI::Code also emits that theme's CSS scoped under [data-theme=X] .code-highlight for each shipped dark theme, so code blocks stay readable when the switcher flips dark.
c.dark_themes	DEFAULT_DARK_THEMES (the built-in daisyUI dark names)	Which theme names count as dark. Intersected with c.themes at render time (#dark_themes_shipped) so only shipped themes emit CSS. Overridable for custom dark themes (e.g. %w[zazu-dark]).

code_theme_dark_class mirrors code_theme_class (String / Class / nil).

Why CSS-only, no JS

The base (light) theme is emitted un-scoped; the dark theme is scoped under [data-theme=X]. daisyUI's [data-theme] selector is more specific than the un-scoped base rule, so the theme switcher restyles code blocks with no JavaScript and no flash. Rouge CSS is inlined per block (not part of the Tailwind build), so the theme-sync invariant is unaffected — a code_theme_dark doesn't need a CSS rebuild.

Test Coverage

• configuration_spec — brand_href default/override; code_theme_dark nil default + override; code_theme_dark_class String/Class/nil; dark_themes frozen default + override; dark_themes_shipped intersection + declaration order + empty case
• shell_spec — brand link href defaults to "/" and follows config.brand_href
• code_spec — dark CSS scoped per shipped dark theme; a non-shipped dark theme (synthwave when unshipped) excluded; base rule coexists with dark rules; byte-identical regression guard when code_theme_dark is nil

Dogfood + docs

• docs/ initializer configured with Github (light) + Monokai (dark) so the light↔dark restyle is visible across all 9 themes
• README documents the three knobs and notes the theme-sync invariant is unaffected
• Install generator template ships the knobs as commented examples (so docs_kit:install and docs-kit new both surface them)

Verification

• bundle exec rspec — 252 examples, 0 failures; Configuration 100% covered
• bundle exec rubocop — no offenses
• End-to-end render check: with themes: %w[light dark synthwave retro dracula] the emitted dark scopes are exactly dark, synthwave, dracula; light/retro excluded; base rule present

Out of scope

• CSP nonce work (in flight on fix/csp-nonce-shell, DocsUI::Shell inline scripts are not nonce-aware — blocked under a nonce-based CSP #2)
• Icon library config (Make DocsUI::Icon's icon library configurable (currently hardcoded to RailsIcons default_library) #3)
• Per-block theme overrides on DocsUI::Code

Closes #16