feat(branches): add force delete flow for local branches

Branch deletion now supports an optional force mode in the Tauri
command, enabling deletion of branches that are not fully merged.
The UI replaces the simple confirm prompt with a dedicated dialog and
auto-escalates to force delete when Git rejects a normal delete.

- Add force flag to delete_branch command and tests
- Implement BranchDeleteConfirmDialog and wire it into App.svelte
- Update branch context menu actions and styling

src/app.css

@@ -1679,6 +1679,12 @@
     color: var(--color-ink);
   }
 
+  .branch-context-menu .menu-separator {
+    height: 1px;
+    margin: 4px 3px;
+    background: var(--color-border-subtle);
+  }
+
   .branch-context-menu button.danger {
     color: #ff9aa8;
   }
@@ -2645,6 +2651,12 @@
     gap: 14px;
     padding: 18px 16px 16px;
   }
+  .branch-delete-body {
+    display: grid;
+    grid-template-columns: auto minmax(0, 1fr);
+    gap: 14px;
+    padding: 18px 16px 16px;
+  }
   .discard-warning-icon {
     display: grid;
     place-items: center;
@@ -2679,6 +2691,11 @@
     white-space: pre-wrap;
     word-break: break-word;
   }
+  .branch-delete-body .discard-target {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+  }
   .discard-warning-text {
     color: #ffb8bf;
     font-weight: 650;