fix(ci): upgrade Flutter 3.35.3 -> 3.44.0 (Dart 3.12.0)

The pubspec requires sdk: ^3.11.5 (Dart >= 3.11.5).
Flutter 3.35.3 ships Dart 3.9.2 which does not satisfy this constraint.
Flutter 3.44.0 ships Dart 3.12.0 which satisfies the constraint.

.gitea/workflows/ci.yml

@@ -7,8 +7,10 @@
 # x86_64 Linux Docker containers, so we use Linux SDK binaries.
 #
 # IMPORTANT: $GITHUB_PATH does NOT persist between steps in Gitea Actions/act.
-# Each step is a separate shell. We install Flutter to a workspace subdirectory
-# and export a shell variable so every step has Flutter on PATH.
+# Each step is a separate shell. We export PATH="$PWD/flutter/bin:$PATH" in
+# every step that uses flutter/dart commands.
+#
+# Flutter 3.44.0 ships Dart 3.12.0, satisfying pubspec sdk: ^3.11.5 constraint.
 
 name: CI
 
@@ -38,23 +40,11 @@ jobs:
       - name: Setup Flutter
         run: |
           set -e
-          echo "=== Diagnosing environment ==="
-          echo "ARCH: $(uname -m)"
-          echo "PWD: $PWD"
-          echo "DF: $(df -h / | tail -1)"
-          echo "DISK FREE: $(du -sh /tmp/ 2>/dev/null || echo unknown)"
-          echo "=== Downloading Flutter SDK ==="
-          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.35.3-stable.tar.xz" \
+          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.44.0-stable.tar.xz" \
             -o /tmp/flutter.tar.xz
-          echo "Download complete, size: $(du -h /tmp/flutter.tar.xz | cut -f1)"
-          echo "=== Extracting ==="
           tar -xf /tmp/flutter.tar.xz -C "$PWD"
-          echo "Extraction complete"
-          ls -la flutter/bin/flutter
-          echo "=== Testing Flutter binary ==="
-          file flutter/bin/flutter || true
-          flutter/bin/flutter --version || echo "FLUTTER FAILED" || true
-          echo "=== Flutter version output ==="
+          export PATH="$PWD/flutter/bin:$PATH"
+          flutter --version
 
       - name: Install dependencies
         run: |
@@ -90,7 +80,7 @@ jobs:
       - name: Setup Flutter
         run: |
           set -e
-          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.35.3-stable.tar.xz" \
+          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.44.0-stable.tar.xz" \
             -o /tmp/flutter.tar.xz
           tar -xf /tmp/flutter.tar.xz -C "$PWD"
           export PATH="$PWD/flutter/bin:$PATH"
@@ -135,7 +125,7 @@ jobs:
       - name: Setup Flutter
         run: |
           set -e
-          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.35.3-stable.tar.xz" \
+          curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.44.0-stable.tar.xz" \
             -o /tmp/flutter.tar.xz
           tar -xf /tmp/flutter.tar.xz -C "$PWD"
           export PATH="$PWD/flutter/bin:$PATH"