Route-based breadcrumb navigation and navbar refinements

#23 in Riparion/riparion-cms — merged 2026-05-31

Summary

• Adds a Breadcrumb component (src/components/breadcrumb/) that reads use_route::<Route>() and renders a contextual trail automatically — no prop drilling required
• Admin pages re-base at Admin (linking to Dashboard); reader pages start from Home; post detail pages show the actual fetched post title instead of a slug-formatted label
• Breadcrumb wired into all 11 chrome-bearing layouts in layouts.rs and into AdminShell for complete coverage
• Replaces the ← Back button on post detail pages

Navbar cleanup:

• Search link replaced with a compact inline search input (Enter to submit); navigates to /search?q=… and updates the URL so the breadcrumb reflects search state
• Home and Archive links removed from navbar — Home is reachable via breadcrumb; Archive moved to a new ArchiveLink sidebar widget that appears after Categories and Tags
• Admin link moved into the user dropdown, remaining permission-gated via admin_landing()

Test plan

• Navigate to /admin/posts, /admin/posts/new, /admin/posts/1/edit — confirm correct admin trail at each level
• Navigate to /category/some-slug, /tag/some-slug — confirm "Home › Categories/Tags › Name"
• Open a post — confirm "Home › [actual post title]" replaces the old ← Back button
• Visit /search with no query — confirm "Home › Search"; type and submit — confirm "Home › Search Results"
• Confirm home page (/) renders no breadcrumb
• Confirm Archive link appears in sidebar below Tags on home and feed pages
• Confirm Admin link appears in user dropdown only for users with admin tokens; absent for regular users
• cargo clippy and cargo fmt --check pass on both targets

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