Enhance PhotoViewer with share and open-in actions, improve reply UI

This commit introduces "Share" and "Open in..." actions to the `PhotoViewScreen`, allowing users to share images via other apps or open them in external viewers.

**Key changes:**

- **PhotoViewer:**
    - Added "Share" and "Open in..." options to the `PhotoViewScreen` dropdown menu.
    - `PhotoViewViewModel`:
        - Implemented `onShareClicked()` and `onOpenInClicked()` to handle these new actions.
        - Added `shareRequest` StateFlow to manage image sharing intents.
        - Introduced `downloadAndStoreImageToCache()` to download and cache images for sharing.
        - `onImageShared()` resets `shareRequest` after sharing.
    - Updated `TopBar` to include the new menu items.
    - Added string resources for "Open in…" and "Share".
- **Reply UI:**
    - `Reply.kt`: Title and summary text now use `TextOverflow.Ellipsis` to prevent long text from breaking the layout.
- **API Model:**
    - `MessagesResponse.kt`: Added `MessagesMarkAsImportantResponse` data class to handle the response for marking messages as important.
- **Data Layer:**
    - `MessagesRepositoryImpl`: Updated `markAsImportant` to correctly map the API response using `MessagesMarkAsImportantResponse`.
- **Minor:**
    - `README.md`: Updated feature checklist for external viewer.
    - `ApplicationModule.kt`: Added experimental Coil API opt-in.

app/src/main/kotlin/dev/meloda/fast/common/di/ApplicationModule.kt

@@ -5,6 +5,7 @@ import android.content.res.Resources
 import android.os.PowerManager
 import androidx.preference.PreferenceManager
 import coil.ImageLoader
+import coil.annotation.ExperimentalCoilApi
 import dev.meloda.fast.MainViewModelImpl
 import dev.meloda.fast.auth.captcha.di.captchaModule
 import dev.meloda.fast.auth.login.di.loginModule
@@ -33,6 +34,7 @@ import org.koin.core.qualifier.qualifier
 import org.koin.dsl.bind
 import org.koin.dsl.module
 
+@OptIn(ExperimentalCoilApi::class)
 val applicationModule = module {
     includes(domainModule)
     includes(
@@ -66,6 +68,7 @@ val applicationModule = module {
         ImageLoader.Builder(get())
             .crossfade(true)
             .build()
+            .also { it.diskCache?.directory?.toFile()?.listFiles() }
     }
 
     singleOf(::LongPollControllerImpl) bind LongPollController::class