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.

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

@@ -354,4 +354,6 @@
     <string name="action_copy_link">Copy link</string>
     <string name="action_copy">Copy</string>
     <string name="action_copy_image">Copy image</string>
+    <string name="action_open_in">Open in…</string>
+    <string name="action_share">Share</string>
 </resources>