feat(shell): two homes — masthead follows brand_href, opt-in app_link topbar link

[mhenrixon]

Closes #62

Summary

DocsUI::Page's masthead "← Home" link hardcoded the host app's root_path route helper — the gem's only call site for it. On a docs site embedded in a bigger application, root_path resolves to the application root (often an authenticated dashboard, not the docs landing), so readers clicking Home were bounced off the docs entirely, and a site's only recourse was overriding the route helper in its own view layer.

This PR gives the chrome two config-driven homes:

• Docs home — the masthead link now follows the existing config.brand_href (default "/"), the same knob the topbar and sidebar brand links already use, and is relabeled "← Docs home". A site that already sets c.brand_href = "/docs" gets the fix on upgrade with zero config change.
• App home — a new opt-in c.app_link = { href:, label: } (default nil → nothing renders). When set, the topbar renders a labeled link back to the hosting app once, right after the brand. Normalized through the existing DocsKit::TopbarLink value object (symbol- or string-keyed Hash, or a TopbarLink); external hrefs open in a new tab with rel=noopener, mirroring DocsUI::TopbarLinks.

Wired into the install path: the generated initializer documents the commented c.app_link example (docs-kit new runs the same generator). README documents both homes.

Backwards compatibility

• app_link unset → the topbar markup is byte-identical to before.
• brand_href default "/" → the masthead points exactly where root_path resolved on standalone sites; only the label changes ("← Home" → "← Docs home"), deliberately.
• Only literal, already-emitted daisyUI class strings — no CSS rebuild or @source change needed.
• Phlex::Rails::Helpers::Routes stays included in Page — consuming sites call route helpers in #content.
• Sites that worked around this by overriding root_path in their view layer can delete the override and, if they want the way back, set c.app_link.

Test plan

• spec/docs_kit/configuration_spec.rb — app_link defaults to nil; Hash (symbol/string keys) normalizes to DocsKit::TopbarLink; an existing TopbarLink passes through.
• spec/docs_ui/shell_spec.rb — unset → no App Home link; set → labeled anchor after the brand anchor; external href → target="_blank" + noopener; relative href → neither.
• spec/docs_ui/page_spec.rb — #home_href defaults to "/" and follows c.brand_href (child-process harness; Page can't load in the Rails-free suite).
• spec/generators/install_generator_spec.rb — the generated initializer contains the commented # c.app_link = example.
• bundle exec rspec: 795 examples, 0 failures, 94.87% line coverage. bundle exec rubocop: clean.

Deviations & judgment calls

• Judgment call: bundle exec rubocop flagged Metrics/ClassLength on app/components/docs_ui/shell.rb (105/100) after adding the ~10-line #app_home_link method — Shell was already at the limit. Exempted shell.rb in .rubocop.yml alongside landing.rb, whose documented rationale (composite renderer of several sections, each a small focused method) applies verbatim. Alternatives rejected: compressing the method to dodge the metric (hurts readability), extracting a one-link component (over-engineering).