Fix strings missing from translations (#959)

## Description
Weblate's Android string parser (aka
https://translate.codeberg.org/projects/wholphin/) doesn't support
direct translations of string arrays.

Many of Wholphin's preference options are defined in string arrays and
therefore could not be translated. So this PR implements the [suggested
workaround](https://docs.weblate.org/en/latest/formats/android.html) to
move the string into individual entries and reference them in arrays.

Also moves some hardcoded strings into `strings.xml` so they can be
translated. Note: virtually all error messages are still hardcoded, but
they are only for exceptional cases, so it's probably not a huge
problem.

A minor bug where the backdrop would show briefly after switching users
is fixed.

### Related issues
Fixes #957

### Testing
Emulator

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/MainActivity.kt

@@ -123,6 +123,9 @@ class MainActivity : AppCompatActivity() {
     @Inject
     lateinit var suggestionsSchedulerService: SuggestionsSchedulerService
 
+    @Inject
+    lateinit var backdropService: BackdropService
+
     // Note: unused but injected to ensure it is created
     @Inject
     lateinit var serverEventListener: ServerEventListener
@@ -247,6 +250,9 @@ class MainActivity : AppCompatActivity() {
                                             }
 
                                             is SetupDestination.AppContent -> {
+                                                LaunchedEffect(Unit) {
+                                                    backdropService.clearBackdrop()
+                                                }
                                                 val current = key.current
                                                 ProvideLocalClock {
                                                     if (UpdateChecker.ACTIVE && appPreferences.autoCheckForUpdates) {