feat(share): inline media preview on single-file share landing

The public share page only rendered media previews for FOLDER shares —
a single-file share got a bare icon + download button, even for images
and videos the browser can play natively.

Backend: resolve the shared file's mime_type + size at read time and
expose them on ShareDto (meta + password-verify endpoints, one shared
enrichment helper). Display-only enrichment: a failed file lookup
leaves the fields None instead of failing the response — the download
endpoint still surfaces the real error.

Frontend: the 'file' view now reuses the folder grid's lazyVideo
(poster-seek + retry) for video and imageRetry for images, with
Range-aware streaming already provided by /api/s/{token}/file/{id}.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/application/dtos/share_dto.rs

@@ -16,6 +16,15 @@ pub struct ShareDto {
     pub created_at: u64,
     pub created_by: String,
     pub access_count: u64,
+    /// File shares only: the shared file's MIME type, resolved at read time
+    /// so anonymous viewers can render an inline media preview (video player
+    /// / image) instead of a bare download button. Absent for folder shares
+    /// and whenever the file lookup fails (display-only enrichment).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub mime_type: Option<String>,
+    /// File shares only: the shared file's size in bytes (see `mime_type`).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub size: Option<u64>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
@@ -49,6 +58,8 @@ impl ShareDto {
             created_at: share.created_at(),
             created_by: share.created_by().to_string(),
             access_count: share.access_count(),
+            mime_type: None,
+            size: None,
         }
     }
 }