feat(web): repo detail page + clickable repo names

[wesbillman]

Summary

• Improves the web empty state layout — centered layout with Sprout branding, clear copy ("This relay is empty"), and "Open in Sprout" CTA
• Fixes "Connect to Relay" button text → "Open in Sprout" everywhere (button, sidebar)
• Adds a /repos/$repoId route with a full repo detail page showing name, description, clone URLs (with copy-to-clipboard + error handling), owner/contributors, external web link, and "Open in Sprout" CTA
• Makes repo names in the list view clickable — they now navigate to the detail page
• Extracts PubkeyAvatar into a shared component reused by both the sidebar and detail page
• useRepo(id) hook reuses the ["repos"] query cache via select — no extra WebSocket connections

Known limitation

• Repo IDs containing dots (e.g. my-lib.core) will 404 on direct URL navigation due to the relay's SPA fallback treating dotted paths as static assets. Tracked as a relay-side follow-up in router.rs.

Test plan

• Navigate to a relay with repos — verify repo names are clickable links
• Click a repo name — verify it navigates to /repos/<id> with correct detail page
• Verify clone URLs display and copy button works (including error feedback)
• Verify back link returns to repo list
• Navigate to a non-existent repo ID — verify "not found" state
• Direct URL navigation to /repos/<id> works (reload the page)
• Verify sidebar still shows "Open in Sprout" button and People section
• Verify empty relay state shows centered layout with correct copy

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