ci: cache pub deps, filter jobs by path, and speed up release builds

CI (every push/PR):
- pub-cache cached keyed on pubspec.lock
- dorny/paths-filter skips the Flutter job for Go-only changes and
  vice versa (skipped required checks count as passing)

Release:
- gomobile installed from the go.mod-pinned x/mobile version instead
  of @latest (reproducible, covered by the setup-go cache; verified
  locally that go.sum already satisfies cmd/gomobile)
- Android bind targets arm/arm64/amd64 only - 386 dropped, amd64 kept
  so the universal APK still runs on x86_64 emulators
- NDK r29 cached; docker prune dropped from the free-space step
- Flutter pinned to 3.41.5 (matches .fvmrc/ci.yml) in both build jobs
  plus pub-cache caching; xcodeproj gem installed only when missing

.github/workflows/ci.yml

@@ -11,9 +11,41 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  changes:
+    name: Detect changed paths
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: read
+    outputs:
+      dart: ${{ steps.filter.outputs.dart }}
+      go: ${{ steps.filter.outputs.go }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Filter paths
+        uses: dorny/paths-filter@v3
+        id: filter
+        with:
+          filters: |
+            dart:
+              - 'lib/**'
+              - 'test/**'
+              - 'assets/**'
+              - 'pubspec.yaml'
+              - 'pubspec.lock'
+              - 'analysis_options.yaml'
+              - 'l10n.yaml'
+              - '.github/workflows/ci.yml'
+            go:
+              - 'go_backend/**'
+              - '.github/workflows/ci.yml'
+
   flutter:
     name: Flutter analyze & test
     runs-on: ubuntu-latest
+    needs: changes
+    if: needs.changes.outputs.dart == 'true'
 
     steps:
       - name: Checkout repository
@@ -27,6 +59,13 @@ jobs:
           channel: "stable"
           cache: true
 
+      - name: Cache pub dependencies
+        uses: actions/cache@v5
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ runner.os }}-${{ hashFiles('pubspec.lock') }}
+          restore-keys: pub-${{ runner.os }}-
+
       - name: Get Flutter dependencies
         run: flutter pub get
 
@@ -39,6 +78,8 @@ jobs:
   go:
     name: Go vet & test
     runs-on: ubuntu-latest
+    needs: changes
+    if: needs.changes.outputs.go == 'true'
 
     defaults:
       run: