Add German, French, Spanish, Italian and Portuguese translations; v0.3.1

Seven UI languages now: English, Dutch, German, French, Spanish, Italian,
Portuguese. Each values-xx/strings.xml carries the full string set (verified
lint-clean). Bump to versionCode 6 / versionName 0.3.1.

Fixes #7

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 = 5
-        versionName = "0.3.0"
+        versionCode = 6
+        versionName = "0.3.1"
 
         // OpenCV native libs are large; ship only the ABIs real phones use.
         ndk {