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/lib/components/MemoryBrowserPanel.svelte

@@ -14,6 +14,7 @@
   interface MemoryFileInfo {
     path: string;
     heading: string | null;
+    last_modified?: string; // Unix timestamp in seconds as a string, optional for backwards compat
   }
 
   interface MemoryFilesResponse {
@@ -65,6 +66,16 @@
     return file.heading ?? getFileName(file.path);
   }
 
+  function formatLastModified(ts: string | undefined): string {
+    if (!ts) return "";
+    const date = new Date(Number(ts) * 1000);
+    return date.toLocaleDateString(undefined, {
+      year: "numeric",
+      month: "short",
+      day: "numeric",
+    });
+  }
+
   async function sendMemoryCommand() {
     const conversationId = get(claudeStore.activeConversationId);
     if (!conversationId) return;
@@ -205,7 +216,12 @@
                     d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z"
                   />
                 </svg>
-                <span class="file-name">{getDisplayName(file)}</span>
+                <div class="file-info">
+                  <span class="file-name">{getDisplayName(file)}</span>
+                  {#if file.last_modified}
+                    <span class="file-date">{formatLastModified(file.last_modified)}</span>
+                  {/if}
+                </div>
               </button>
             {/each}
           </div>
@@ -416,7 +432,7 @@
 
   .file-item {
     display: flex;
-    align-items: center;
+    align-items: flex-start;
     gap: 0.75rem;
     padding: 0.75rem 1rem;
     background: var(--bg-secondary);
@@ -445,6 +461,13 @@
     flex-shrink: 0;
   }
 
+  .file-info {
+    display: flex;
+    flex-direction: column;
+    gap: 0.125rem;
+    overflow: hidden;
+  }
+
   .file-name {
     font-size: 0.875rem;
     font-weight: 500;
@@ -453,6 +476,15 @@
     white-space: nowrap;
   }
 
+  .file-date {
+    font-size: 0.75rem;
+    color: var(--text-secondary);
+  }
+
+  .file-item.active .file-date {
+    color: rgba(255, 255, 255, 0.75);
+  }
+
   .file-viewer {
     display: flex;
     flex-direction: column;