fix(ci): use [self-hosted, linux] runner labels for lint and test jobs

The pipeline was failing because Lint and Test jobs used runs-on:
ubuntu-latest which 'act' does not recognize as a local runner label.
On the Mac build server (Magrathea), these jobs were routed to the
macOS runner where Docker containers failed to run Flutter correctly.

Changed:
- lint: ubuntu-latest -> [self-hosted, linux]
- test: ubuntu-latest -> [self-hosted, linux]
- build-android: ubuntu-latest (unchanged)
- build-ios: [self-hosted, macos] (unchanged)

The Linux runner should pick up lint/test while Magrathea handles iOS
builds as intended.

.gitea/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
   # ── Stage 1: Lint & static analysis ──────────────────────────────
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -50,7 +50,7 @@ jobs:
   # ── Stage 2: Unit & widget tests ─────────────────────────────────
   test:
     name: Test
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     needs: [lint]
     steps:
       - name: Checkout