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/model/src/main/kotlin/dev/meloda/fast/model/api/responses/MessagesResponse.kt

@@ -58,3 +58,15 @@ data class MessagesSendResponse(
     @Json(name = "message_id") val messageId: Long,
     @Json(name = "cmid") val cmId: Long
 )
+
+@JsonClass(generateAdapter = true)
+data class MessagesMarkAsImportantResponse(
+    @Json(name = "marked") val marked: List<Mark>
+) {
+    @JsonClass(generateAdapter = true)
+    data class Mark(
+        @Json(name = "cmid") val cmId: Long,
+        @Json(name = "message_id") val messageId: Long,
+        @Json(name = "peer_id") val peerId: Long
+    )
+}