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/main/java/com/m57/hermescontrol/util/LocaleContextWrapper.kt

@@ -0,0 +1,73 @@
+package com.m57.hermescontrol.util
+
+import android.content.Context
+import android.content.ContextWrapper
+import android.os.Build
+import java.util.Locale
+
+/**
+ * Static helpers for applying a user-selected in-app display language.
+ *
+ * Hermes Mobile's [com.m57.hermescontrol.MainActivity] extends the plain
+ * `ComponentActivity` (not `AppCompatActivity`), so we apply the locale through
+ * `attachBaseContext` + [ContextWrapper.wrap] rather than relying on
+ * AppCompatDelegate. Works on minSdk 26.
+ */
+object LocaleContextWrapper {
+    const val SYSTEM_LANGUAGE = "system"
+
+    /**
+     * Resolve a language code ("system", "en", "ko", …) into a [Locale].
+     *
+     * "system" returns the device's default locale (so passing it through
+     * [wrap] is effectively a no-op). Codes containing a region separator
+     * ("zh-rCN", "pt-BR") are split into language + country.
+     */
+    fun localeForCode(code: String): Locale =
+        when {
+            code.isEmpty() || code == SYSTEM_LANGUAGE -> Locale.getDefault()
+            code.contains("-r", ignoreCase = true) -> {
+                val parts = code.split("-r", ignoreCase = true)
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            code.contains("-") -> {
+                val parts = code.split("-")
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            code.contains("_") -> {
+                val parts = code.split("_")
+                Locale(parts[0], parts.getOrElse(1) { "" })
+            }
+            else -> Locale(code)
+        }
+
+    /**
+     * Wrap [base] so the configuration carries [locale].
+     *
+     * On API 33+ we use [Context.createConfigurationContext] which is the only
+     * reliable path once the deprecated `Configuration.locale` / `setLocale`
+     * APIs were removed from the public surface.
+     */
+    fun wrap(
+        base: Context,
+        locale: Locale,
+    ): Context {
+        val config = base.resources.configuration
+        val newConfig =
+            android.content.res.Configuration(config).apply {
+                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+                    setLocales(android.os.LocaleList(locale))
+                } else {
+                    @Suppress("DEPRECATION")
+                    setLocale(locale)
+                }
+            }
+        return base.createConfigurationContext(newConfig)
+    }
+
+    /** Convenience: wrap [base] using a stored language code. */
+    fun wrapWithCode(
+        base: Context,
+        code: String,
+    ): Context = wrap(base, localeForCode(code))
+}