feat: add mobile smoke tests and QR code for mobile URL (#2403)

* feat: add mobile smoke tests and QR code for mobile URL

#2396: Add scripts/mobile-smoke.sh that launches the compiled binary on
loopback ports and verifies the mobile surface through real HTTP requests:
- Token auth (401/200, Bearer, query param, approval 404)
- Insecure mode (no token required)
- Binding warnings (0.0.0.0, LAN URL hint)

Add mobile-smoke job to CI workflow.

#2397: Add --qr flag to 'codewhale serve --mobile' that renders a
terminal QR code for the mobile URL. Uses the LAN IP when available,
falls back to 127.0.0.1. Adds qrcode crate (pure Rust, no C deps).

* fix: address review feedback on mobile smoke tests

- Fix Test Group 3 subprocess capture: use temp file instead of command
  substitution to avoid hanging and subshell variable isolation
- Allow BINARY path to be overridden via BINARY env var
- Add libdbus-1-dev system dependency to CI job for ubuntu build

* fix: pass auth header in mobile smoke status helper

* fix: send approval JSON in mobile smoke

---------

Co-authored-by: Hu Qiantao <huqiantao@HudeMacBook-Air.local>
Co-authored-by: Hunter B <hmbown@gmail.com>

.github/workflows/ci.yml

@@ -103,6 +103,27 @@ jobs:
         if: runner.os == 'Linux'
         run: echo "Linux npm wrapper smoke runs on CNB for mirrored first-party branches."
 
+  mobile-smoke:
+    name: Mobile runtime smoke
+    if: github.event_name != 'schedule'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - name: Install Linux system dependencies
+        run: |
+          for i in 1 2 3 4 5; do
+            sudo apt-get update && break
+            echo "apt-get update failed (attempt $i); retrying in 15s"
+            sleep 15
+          done
+          sudo apt-get install -y libdbus-1-dev pkg-config
+      - uses: Swatinem/rust-cache@v2
+        with:
+          cache-bin: false
+      - name: Run mobile smoke tests
+        run: ./scripts/mobile-smoke.sh
+
   # Check documentation builds without warnings
   docs:
     name: Documentation