feat(ui): add collapsible Worktree and Tags panels to left sidebar

Add two new sidebar components (WorktreePanel, TagsPanel) and integrate them
into the compact left navigation layout:

- Wire up imports and rendering in App.svelte, including collapse toggles and
  resize handles for both panels. Persisted heights (localStorage) and keyboard
  resizing are supported; heights are clamped between 80 and 420px with a 140px
  default.
- Extend buildLeftSidebarRows and left-sidebar grid/template styles to include
  the new panels and reduce panel-handle thickness. Add extensive CSS for the
  compact accordion navigation, worktree and tags lists.
- Move tag and worktree management UI out of BranchPanel (remove tag props),
  and add dedicated handlers in App.svelte for the new panels.
- Refactor worktree loading: introduce a worktreeLoadId to guard async
  refreshWorktrees() calls and avoid race conditions. refreshWorktrees(path?)
  now takes an optional path and updates worktree state only when the request
  is still relevant.
- Small behavioral tweaks: stash panel default collapsed state changed to true
  and the explorer resize-handle visibility condition adjusted.

This commit only adds the UI/UX integration and local persistence for the new
panels and their resizing/refresh behavior.

src/lib/components/StashPanel.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { Archive, ChevronDown, ChevronRight, Download, Trash2, Upload } from "@lucide/svelte";
+  import { Archive, ChevronDown, ChevronRight, Download, Plus, Trash2, Upload } from "@lucide/svelte";
   import type { GitStash } from "../types";
 
   interface Props {
@@ -28,6 +28,7 @@
     onToggleCollapsed = () => {},
   }: Props = $props();
 
+  let createOpen = $state(false);
   let message = $state("");
   let includeUntracked = $state(true);
 
@@ -43,11 +44,13 @@
 
 <section class="panel stash-panel overflow-hidden" class:collapsed aria-label="Git stash">
   <div class="section-head">
-    <div>
-      <span class="eyebrow">Stash</span>
-      <h2 class="mt-0.5 text-ink text-base font-bold leading-tight">Shelved changes</h2>
-    </div>
+    <h2 class="sidebar-section-title"><Archive size={16} aria-hidden="true" />Stashes</h2>
     <div class="stash-head-actions">
+      <button class="stash-toggle" type="button" title="Create stash" aria-label="Create stash"
+        disabled={!hasRepository || isBusy || changedCount === 0}
+        onclick={() => { createOpen = !createOpen; if (collapsed) { createOpen = true; onToggleCollapsed(); } }}>
+        <Plus size={14} aria-hidden="true" />
+      </button>
       <span class="pill pill-count">{stashes.length}</span>
       <button
         class="stash-toggle panel-collapse-toggle"
@@ -71,12 +74,14 @@
   {:else if !hasRepository}
     <div class="blank-state">No repository loaded.</div>
   {:else}
+    {#if createOpen}
     <div class="stash-create">
       <input
         class="stash-input"
         type="text"
         bind:value={message}
         placeholder="Optional message"
+        aria-label="Stash message"
         disabled={isBusy || changedCount === 0}
         onkeydown={(event) => {
           if (event.key === "Enter" && changedCount > 0 && !isBusy) submitPush();
@@ -98,6 +103,8 @@
       </button>
     </div>
 
+    {/if}
+
     {#if stashes.length === 0}
       <div class="blank-state stash-empty">No stashes saved.</div>
     {:else}