Internationalization: string resources + Dutch translation; v0.3.0

Move all user-facing strings into res/values/strings.xml (English base) and
add res/values-nl/strings.xml (Dutch). Composables use stringResource;
coroutine-set and non-composable strings (ViewModel, PaperlessClient) resolve
via Context.getString. Mark app_name translatable=false. Bump to 0.3.0.

Fixes #5

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/build.gradle.kts

@@ -24,8 +24,8 @@ android {
         applicationId = "eu.geyskens.pdfscan"
         minSdk = 26
         targetSdk = 35
-        versionCode = 4
-        versionName = "0.2.0"
+        versionCode = 5
+        versionName = "0.3.0"
 
         // OpenCV native libs are large; ship only the ABIs real phones use.
         ndk {