fix(repo-icon): keep a renamed fork's own owner avatar (#12271)

* fix(repo-icon): keep a renamed fork's own owner avatar

Fork repos always took the upstream owner's avatar, so a renamed fork
showed its parent project's logo. Same-name forks (personal copies)
still prefer the upstream owner; renamed forks now keep their origin
owner across auto-detect, the startup backfill, and the settings
avatar refresh.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(repo-icon): re-read repo state before backfill avatar write

The startup backfill computed icon updates from a pre-loop snapshot, so
an icon chosen in settings while the upstream/origin probes were pending
could be clobbered. Re-read the repo after the probes and only migrate
an icon that is still the auto-detected GitHub avatar.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* refactor(repo-icon): own the fork avatar rule in one shared selector

The renamed-fork rule was written out twice — once in the main-process
auto-detect and once in the renderer refresh — so the two copies could
drift. Move it next to `githubAvatarIcon` as `githubAvatarSlug`, which
collapses the renderer resolver to a single unbranched path.

Also stop swallowing a rejected origin probe: it cannot tell a renamed
fork from a same-name one, so degrading to the upstream owner would flip
a renamed fork's stored avatar back to the parent's. Letting it propagate
keeps the stored icon, matching how the non-fork path already behaved.

Adds coverage for the startup backfill, the third decision point the fix
claims, which had none.

* test(repo-icon): cover pending backfill icon change

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Brennan Benson <79079362+brennanb2025@users.noreply.github.com>

src/main/github/client.ts

@@ -1669,7 +1669,8 @@ export async function getRepoSlug(
 
 /**
  * Resolve a fork's upstream/parent owner/repo, or null when not a fork.
- * Why: a fork's `origin` is the personal copy, so repo identity (avatar) should prefer upstream.
+ * Why: drives the fork indicator, and a same-name fork's avatar prefers the
+ * upstream owner (a renamed fork keeps its own owner).
  * Best-effort: any failure (offline, unauthed, non-GitHub) resolves to null.
  */
 export async function getRepoUpstream(