chore(#322): upload build artifacts + build on dev pushes

Two diagnostics faults from #313, fixed together:

1. Dead push trigger. build.yml and flutter_dart.yml declared
   `push: branches: [main]`, but no `main` branch exists in this
   repo (default is `dev`), so the push half of every trigger had
   never fired. Nothing built on merge. Added `dev` to both; `main`
   is created separately after this merges.

2. No artifacts. All six platform jobs stopped at `flutter build`,
   so every binary CI produced was discarded with the runner. Added
   actions/upload-artifact to the four jobs that produce something
   installable or servable:

     android  build/app/outputs/flutter-apk/app-release.apk
     windows  build/windows/x64/runner/Release/
     linux    build/linux/x64/release/bundle/
     web      build/web/

   ios and macos are compile checks only and upload nothing: ios is
   built --no-codesign so it is not installable, and macos is kept
   as a compile check per the owner's decision on #321.

Artifacts carry run number + commit SHA so a build traces back to a
commit, and use if-no-files-found: error so a wrong path fails loudly
rather than silently publishing an empty artifact.

Android artifacts are debug-signed for now; real signing is Phase 5.
The web artifact is the plain `flutter build web` output, not the
build_pipe versioned build used for deploys; Phase 4 addresses that.

Part of epic #312, plan #321 (Phase 1, owner-approved).

.github/workflows/build.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - main
+      - dev
   pull_request:
 
 jobs:
@@ -30,6 +31,13 @@ jobs:
             ${{ runner.os }}-gradle-
       - run: flutter pub get
       - run: flutter build apk --release --no-pub
+      - name: Upload APK
+        uses: actions/upload-artifact@v4
+        with:
+          name: offband-android-r${{ github.run_number }}-${{ github.sha }}
+          path: build/app/outputs/flutter-apk/app-release.apk
+          if-no-files-found: error
+          retention-days: 14
 
   ios:
     runs-on: macos-latest
@@ -54,6 +62,13 @@ jobs:
         run: sudo apt-get update && sudo apt-get install -y clang cmake ninja-build pkg-config libgtk-3-dev
       - run: flutter pub get
       - run: flutter build linux --release --no-pub
+      - name: Upload Linux bundle
+        uses: actions/upload-artifact@v4
+        with:
+          name: offband-linux-r${{ github.run_number }}-${{ github.sha }}
+          path: build/linux/x64/release/bundle/
+          if-no-files-found: error
+          retention-days: 14
 
   macos:
     runs-on: macos-latest
@@ -76,6 +91,13 @@ jobs:
           cache: true
       - run: flutter pub get
       - run: flutter build web --release --no-pub
+      - name: Upload web build
+        uses: actions/upload-artifact@v4
+        with:
+          name: offband-web-r${{ github.run_number }}-${{ github.sha }}
+          path: build/web/
+          if-no-files-found: error
+          retention-days: 14
 
   windows:
     runs-on: windows-latest
@@ -87,3 +109,10 @@ jobs:
           cache: true
       - run: flutter pub get
       - run: flutter build windows --release --no-pub
+      - name: Upload Windows build
+        uses: actions/upload-artifact@v4
+        with:
+          name: offband-windows-r${{ github.run_number }}-${{ github.sha }}
+          path: build/windows/x64/runner/Release/
+          if-no-files-found: error
+          retention-days: 14

.github/workflows/flutter_dart.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
       - main
+      - dev
 
 jobs:
   analyze: