ci: run on the desktop runner (runs-on: desktop)

Move jobs from gitdan-ci's ubuntu-latest to the desktop runner label,
the default CI runner per daniel/gitdan#112, and fix comments and docs
that named the old runner.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MfBCVP2GbKqXSmmJXRPnXP

.gitea/workflows/ci.yaml

@@ -13,7 +13,7 @@ jobs:
   # Run cargo test
   test:
     name: Test Suite
-    runs-on: ubuntu-latest
+    runs-on: desktop
     timeout-minutes: 30
     steps:
       - name: Checkout sources
@@ -38,7 +38,7 @@ jobs:
   # Run cargo clippy -- -D warnings
   clippy_check:
     name: Clippy
-    runs-on: ubuntu-latest
+    runs-on: desktop
     timeout-minutes: 30
     steps:
       - name: Checkout sources
@@ -65,7 +65,7 @@ jobs:
   # Run cargo fmt --all -- --check
   format:
     name: Format
-    runs-on: ubuntu-latest
+    runs-on: desktop
     timeout-minutes: 30
     steps:
       - name: Checkout sources