feat: add channel message support and refactor UI components

- Implement `VkChannelMessage` domain and data models for channel message attachments
- Add `CHANNEL_MESSAGE` to `AttachmentType` and map it to relevant UI resources and strings
- Refactor `Sticker` and `Gift` composables to accept URL strings instead of domain objects for better decoupling
- Simplify `AppTheme` by removing redundant color animations and passing the color scheme directly to `MaterialExpressiveTheme`
- Update `LoginScreen` to include a theme toggle (classic vs. light) and improve back-button behavior by resetting error states
- Bump VK API version from 5.238 to 5.263
- Adjust layout and padding for sticker and gift attachment previews

core/ui/src/main/kotlin/dev/meloda/fast/ui/theme/AppTheme.kt

@@ -58,7 +58,7 @@ fun AppTheme(
 ) {
     val context = LocalContext.current
 
-    val colorScheme: ColorScheme = when {
+    val colorScheme: ColorScheme = predefinedColorScheme ?: when {
         useDynamicColors && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S -> {
             if (useDarkTheme) dynamicDarkColorScheme(context)
             else dynamicLightColorScheme(context)
@@ -82,10 +82,6 @@ fun AppTheme(
         }
     }
 
-    val colorPrimary by animateColorAsState(colorScheme.primary)
-    val colorSurface by animateColorAsState(colorScheme.surface)
-    val colorBackground by animateColorAsState(colorScheme.background)
-
     val typography = if (useSystemFont) {
         MaterialTheme.typography
     } else {
@@ -118,12 +114,7 @@ fun AppTheme(
     }
 
     MaterialExpressiveTheme(
-        colorScheme = (predefinedColorScheme ?: colorScheme)
-            .copy(
-                primary = colorPrimary,
-                background = colorBackground,
-                surface = colorSurface
-            ),
+        colorScheme = colorScheme,
         typography = typography,
         content = content
     )

core/ui/src/main/res/values/strings.xml

@@ -88,6 +88,7 @@
     <string name="message_attachments_video_message">Video message</string>
     <string name="message_attachments_group_sticker">Group sticker</string>
     <string name="message_attachments_sticker_pack_preview">Sticker pack preview</string>
+    <string name="message_attachments_channel_message">Channel message</string>
 
     <string name="chat_interaction_uploading_file">Uploading file</string>
     <string name="chat_interaction_uploading_photo">Uploading photo</string>