fix: browse and install from the catalog without GitHub sign-in

[MotherSphere]

Why

The UI says everywhere that signing in to GitHub is optional ("Public GitHub API (60 req/h)", "Connect GitHub (optional)" in the welcome flow), but the catalog lived inside GitHubState::Connected: every fetch path required a token and the repositories were only stored while signed in. A first launch showed an empty store until OAuth was done, and signing out lost the catalog.

Changes

• The catalog moves out of GitHubState into a field of its own on the App: it is public data, independent of the session. Signing in only raises the rate limit.
• Boot: the on-disk repository cache is loaded immediately (an instant grid that also works offline), then a fetch always starts, authenticated when a token is saved and anonymous otherwise. Cached icons are decoded at boot instead of waiting for the first network round trip.
• Refresh works while signed out (anonymous fetch). Sign-in completion sets is_fetching_repos like every other fetch path, which closes a double-fetch window.
• An error toast only when the catalog is EMPTY: with a cached catalog on screen, a network failure goes to the status line. Otherwise every offline boot would raise an error toast.

Testing

• cargo build and clippy --all-targets clean, 88 tests pass.
• End-to-end anonymous mode: launched with empty XDG directories and an unreachable keyring (so no token), the catalog fills from the anonymous fetch (Grape, D1Gg2r and orCAL detected in the logs).