Add path filters to CI and build workflows

CI skips on changes to platform dirs (ios, linux, macos, web, windows),
docs, and README. Build skips on test-only changes since tests don't
affect the release APK.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/build.yml

@@ -3,6 +3,13 @@ name: Build APK
 on:
   push:
     branches: [main]
+    paths:
+      - "lib/**"
+      - "pubspec.yaml"
+      - "pubspec.lock"
+      - "android/**"
+      - "assets/**"
+      - ".forgejo/workflows/build.yml"
 
 jobs:
   build: