feat(ui): add worktree management toggle to branch panel

Introduces a dedicated section and interactive button for managing repository worktrees within the branch list view. This improves visibility into the overall state of local repositories by grouping related controls.

- Adds visual styling and structure for the new worktree group toggle in CSS.
- Replaces the old "Manage worktrees" button with a more descriptive, styled component in BranchPanel.svelte.

src/lib/components/BranchPanel.svelte

@@ -379,16 +379,6 @@
       <h2 class="mt-0.5 text-ink text-base font-bold leading-tight">Refs</h2>
     </div>
     <div class="branch-head-actions">
-      <button
-        class="branch-create-toggle"
-        type="button"
-        onclick={onManageWorktrees}
-        disabled={!hasRepository || isBusy}
-        title="Manage worktrees"
-        aria-label="Manage worktrees"
-      >
-        <HardDrive size={14} aria-hidden="true" />
-      </button>
       <button
         class="branch-create-toggle"
         type="button"
@@ -421,8 +411,6 @@
     <!-- collapsed -->
   {:else if !hasRepository}
     <p class="blank-state">Open a repository to list branches.</p>
-  {:else if branches.length === 0 && tags.length === 0}
-    <p class="blank-state">No branches returned.</p>
   {:else}
     <div class="branch-list overflow-auto p-2 flex flex-col gap-0">
       {#if createOpen}
@@ -659,6 +647,21 @@
           {/if}
         {/if}
       </div>
+
+      <div class="branch-group">
+        <button
+          class="branch-group-toggle worktree-group-toggle"
+          type="button"
+          onclick={onManageWorktrees}
+          disabled={isBusy}
+          aria-haspopup="dialog"
+          title="Manage repository worktrees"
+        >
+          <HardDrive size={14} aria-hidden="true" />
+          <span>Worktrees</span>
+          <span class="branch-group-count">Manage</span>
+        </button>
+      </div>
     </div>
   {/if}