feat(status-panel): enhance file staging and un-staging functionality

This update improves the file staging and un-staging process by allowing
multiple files to be selected and acted upon simultaneously. The user
interface has been enhanced to provide clear feedback on the number of
selected files and their respective statuses.

- Added support for staging and un-staging multiple files at once
- Introduced visual indicators for selected files in the UI
- Improved event handling for file selection and actions

src/app.css

@@ -1320,6 +1320,7 @@
   .file-row { display: grid; gap: 8px; padding: 10px; border: 1px solid var(--color-border-subtle); border-radius: 8px; background: var(--color-surface-raised); }
   .file-row + .file-row { margin-top: 6px; }
   .file-row.selected { border-color: rgba(90,140,248,0.36); background: rgba(90,140,248,0.1); }
+  .file-row.active { box-shadow: inset 3px 0 0 rgba(65,209,255,0.7); }
 
   .file-title-button {
     justify-content: flex-start;
@@ -1370,6 +1371,13 @@
     border-bottom: 1px solid var(--color-border-subtle);
     background: var(--color-surface-dim);
   }
+  .status-selection-count {
+    padding: 0 7px;
+    color: var(--color-ink-dim);
+    font-size: 11.5px;
+    font-weight: 800;
+    white-space: nowrap;
+  }
 
   /* --- Stash panel --- */