feat: surface memory file last-modified timestamps in MemoryBrowserPanel (#230)

Extends MemoryFileInfo with last_modified (Unix timestamp) populated via
fs::metadata on native platforms, and displays a formatted date in the
file list as secondary text.

src-tauri/src/commands.rs

@@ -1464,6 +1464,7 @@ pub async fn close_application(app_handle: AppHandle) -> Result<(), String> {
 pub struct MemoryFileInfo {
     pub path: String,
     pub heading: Option<String>,
+    pub last_modified: Option<String>, // Unix timestamp in seconds as a string
 }
 
 #[derive(serde::Serialize)]
@@ -1535,7 +1536,11 @@ async fn list_memory_files_via_wsl() -> Result<MemoryFilesResponse, String> {
     let mut files = Vec::new();
     for path in paths {
         let heading = read_wsl_file_first_heading(&path);
-        files.push(MemoryFileInfo { path, heading });
+        files.push(MemoryFileInfo {
+            path,
+            heading,
+            last_modified: None,
+        });
     }
 
     Ok(MemoryFilesResponse { files })
@@ -1605,14 +1610,23 @@ async fn list_memory_files_native() -> Result<MemoryFilesResponse, String> {
     // Sort files alphabetically
     memory_paths.sort();
 
-    // Read first heading from each file
+    // Read first heading and modification time from each file
     let files = memory_paths
         .into_iter()
         .map(|path| {
             let heading = fs::read_to_string(&path)
                 .ok()
                 .and_then(|content| extract_first_heading(&content));
-            MemoryFileInfo { path, heading }
+            let last_modified = fs::metadata(&path)
+                .ok()
+                .and_then(|m| m.modified().ok())
+                .and_then(|t| t.duration_since(std::time::UNIX_EPOCH).ok())
+                .map(|d| d.as_secs().to_string());
+            MemoryFileInfo {
+                path,
+                heading,
+                last_modified,
+            }
         })
         .collect();