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/domain/src/main/kotlin/dev/meloda/fast/domain/util/ConvoMapper.kt

@@ -598,6 +598,7 @@ private fun getAttachmentIconByType(attachmentType: AttachmentType): UiImage? {
         AttachmentType.VIDEO_MESSAGE -> null
         AttachmentType.GROUP_CHAT_STICKER -> R.drawable.ic_sticker_fill_round_24
         AttachmentType.STICKER_PACK_PREVIEW -> null
+        AttachmentType.CHANNEL_MESSAGE -> null
     }?.let(UiImage::Resource)
 }
 
@@ -687,6 +688,7 @@ fun getAttachmentUiText(
         AttachmentType.VIDEO_MESSAGE -> R.string.message_attachments_video_message
         AttachmentType.GROUP_CHAT_STICKER -> R.string.message_attachments_group_sticker
         AttachmentType.STICKER_PACK_PREVIEW -> R.string.message_attachments_sticker_pack_preview
+        AttachmentType.CHANNEL_MESSAGE -> R.string.message_attachments_channel_message
     }.let(UiText::Resource)
 }