feat(ui): enhance panel resizing and state persistence

This update introduces functionality for resizing various panels in the UI, including the left sidebar, branch panel, stash panel, and file history. It also adds state persistence for panel sizes and collapsed states using local storage, improving user experience by maintaining preferences across sessions.

- Implemented resizing functionality for multiple UI panels
- Added local storage support for panel dimensions and collapsed states
- Enhanced accessibility features for panel controls

src/lib/components/StashPanel.svelte

@@ -11,6 +11,8 @@
     onApply: (stash: GitStash) => void;
     onPop: (stash: GitStash) => void;
     onDrop: (stash: GitStash) => void;
+    collapsed?: boolean;
+    onToggleCollapsed?: () => void;
   }
 
   let {
@@ -22,11 +24,12 @@
     onApply = () => {},
     onPop = () => {},
     onDrop = () => {},
+    collapsed = false,
+    onToggleCollapsed = () => {},
   }: Props = $props();
 
   let message = $state("");
   let includeUntracked = $state(true);
-  let open = $state(false);
 
   function submitPush() {
     onPush(message, includeUntracked);
@@ -38,31 +41,32 @@
   }
 </script>
 
-<section class="panel stash-panel overflow-hidden" class:collapsed={!open} aria-label="Git stash">
+<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>
     <div class="stash-head-actions">
+      <span class="pill pill-count">{stashes.length}</span>
       <button
-        class="stash-toggle"
+        class="stash-toggle panel-collapse-toggle"
         type="button"
-        onclick={() => { open = !open; }}
-        aria-expanded={open}
-        title={open ? "Collapse stash panel" : "Expand stash panel"}
+        onclick={onToggleCollapsed}
+        aria-expanded={!collapsed}
+        title={collapsed ? "Expand stash panel" : "Collapse stash panel"}
+        aria-label={collapsed ? "Expand stash panel" : "Collapse stash panel"}
       >
-        {#if open}
-          <ChevronDown size={14} aria-hidden="true" />
-        {:else}
+        {#if collapsed}
           <ChevronRight size={14} aria-hidden="true" />
+        {:else}
+          <ChevronDown size={14} aria-hidden="true" />
         {/if}
       </button>
-      <span class="pill pill-count">{stashes.length}</span>
     </div>
   </div>
 
-  {#if !open}
+  {#if collapsed}
     <!-- collapsed -->
   {:else if !hasRepository}
     <div class="blank-state">No repository loaded.</div>