feat: in-app display language switcher (Settings → Appearance) (#604)

Add a Language selector to Settings → Appearance that overrides the app
display language independent of the device locale. Persists a BCP-47 code
in ServerStoreState.appLanguage; MainActivity.attachBaseContext applies it
via a ContextWrapper (works on minSdk 26, no AppCompat dependency needed
since the Activity is a plain ComponentActivity). Selecting a language
recreates the activity so resources re-resolve immediately.

Includes System default / English / 한국어 options, plus a unit test for
the locale-code resolver.

Note: the new string keys (settings_item_language, language_system,
language_english, language_korean) should be backfilled into values-ko
once PR #602 lands.

Co-authored-by: hy4ri <hy4ri@users.noreply.github.com>

app/src/test/java/com/m57/hermescontrol/util/LocaleContextWrapperTest.kt

@@ -0,0 +1,45 @@
+package com.m57.hermescontrol.util
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.util.Locale
+
+class LocaleContextWrapperTest {
+    @Test
+    fun systemReturnsDeviceDefault() {
+        assertEquals(Locale.getDefault(), LocaleContextWrapper.localeForCode("system"))
+    }
+
+    @Test
+    fun emptyReturnsDeviceDefault() {
+        assertEquals(Locale.getDefault(), LocaleContextWrapper.localeForCode(""))
+    }
+
+    @Test
+    fun bareLanguageCode() {
+        val locale = LocaleContextWrapper.localeForCode("ko")
+        assertEquals("ko", locale.language)
+        assertEquals("", locale.country)
+    }
+
+    @Test
+    fun regionSeparator_r() {
+        val locale = LocaleContextWrapper.localeForCode("zh-rCN")
+        assertEquals("zh", locale.language)
+        assertEquals("CN", locale.country)
+    }
+
+    @Test
+    fun regionSeparator_hyphen() {
+        val locale = LocaleContextWrapper.localeForCode("pt-BR")
+        assertEquals("pt", locale.language)
+        assertEquals("BR", locale.country)
+    }
+
+    @Test
+    fun regionSeparator_underscore() {
+        val locale = LocaleContextWrapper.localeForCode("en_US")
+        assertEquals("en", locale.language)
+        assertEquals("US", locale.country)
+    }
+}