fix(ci): pin runners to [self-hosted, linux, x64] to avoid Magrathea ARM64 runner

Magrathea (ARM64 Mac) consistently fails at Flutter setup due to x86_64
container emulation issues. Pinning all jobs to [self-hosted, linux, x64]
ensures they only run on the working x86_64 runner.

.gitea/workflows/ci.yml

@@ -2,18 +2,14 @@
 # Triggers on push and pull_request
 # Stages: lint -> test -> build (Android + iOS) -> artifacts
 #
-# NOTE: Gitea's `act` runner registers as `ubuntu-latest`. Jobs MUST use
-# `runs-on: ubuntu-latest` to be picked up. The runner executes jobs inside
-# x86_64 Linux Docker containers, so we use Linux SDK binaries.
+# IMPORTANT: Our self-hosted runners have different architectures:
+#   - "runner"       = x86_64 Linux (works for all jobs)
+#   - "Magrathea"    = ARM64 Mac (x86_64 Docker emulation fails)
+# We pin ALL jobs to [self-hosted, linux, x64] to avoid the Magrathea runner.
 #
 # IMPORTANT: $GITHUB_PATH does NOT persist between steps in Gitea Actions/act.
 # Each step is a separate shell. We export PATH="$PWD/flutter/bin:$PATH" in
 # every step that uses flutter/dart commands.
-#
-# subosito/flutter-action works on some runners but fails on others (e.g.
-# Magrathea). We use a direct Flutter SDK download for reliability across
-# all runners. For build-android we need manual JDK + Android SDK setup
-# since actions/setup-java is GitHub-specific.
 
 name: CI
 
@@ -38,7 +34,7 @@ jobs:
   # ── Stage 1: Lint & static analysis ──────────────────────────────
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, x64]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -77,7 +73,7 @@ jobs:
   # ── Stage 1 (parallel): Unit & widget tests ──────────────────────
   test:
     name: Test
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, x64]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -110,7 +106,7 @@ jobs:
   # ── Stage 3: Android build ──────────────────────────────────────
   build-android:
     name: Build Android
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, x64]
     needs: [lint, test]
     steps:
       - name: Checkout
@@ -206,7 +202,7 @@ jobs:
   # ── Stage 3 (parallel): iOS build ───────────────────────────────
   build-ios:
     name: Build iOS
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, x64]
     needs: [lint, test]
     steps:
       - name: Checkout