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/ExplorerPanel.svelte

@@ -38,6 +38,8 @@
     onSelectNode: (node: ExplorerNode) => void;
     onOpenFile: (node: ExplorerNode) => void;
     onBlame: (node: ExplorerNode) => void;
+    collapsed?: boolean;
+    onToggleCollapsed?: () => void;
   }
 
   let {
@@ -53,6 +55,8 @@
     onSelectNode = () => {},
     onOpenFile = () => {},
     onBlame = () => {},
+    collapsed = false,
+    onToggleCollapsed = () => {},
   }: Props = $props();
 
   let contextNode = $state<ExplorerNode | null>(null);
@@ -203,7 +207,7 @@
 
 <svelte:window on:click={closeFileContextMenu} on:keydown={handleWindowKeydown} on:contextmenu|capture={closeFileContextMenu} />
 
-<section class="panel explorer-panel grid grid-rows-[auto_1fr] overflow-hidden" aria-label="File explorer">
+<section class="panel explorer-panel grid grid-rows-[auto_1fr] overflow-hidden" class:collapsed aria-label="File explorer">
   <div class="section-head">
     <div>
       <span class="eyebrow">Explorer</span>
@@ -231,10 +235,26 @@
         <Folder size={14} aria-hidden="true" />
       </button>
       <span class="pill pill-count">{repoFiles.length}</span>
+      <button
+        class="explorer-bulk-button panel-collapse-toggle"
+        type="button"
+        onclick={onToggleCollapsed}
+        aria-expanded={!collapsed}
+        title={collapsed ? "Expand file explorer" : "Collapse file explorer"}
+        aria-label={collapsed ? "Expand file explorer panel" : "Collapse file explorer panel"}
+      >
+        {#if collapsed}
+          <ChevronRight size={14} aria-hidden="true" />
+        {:else}
+          <ChevronDown size={14} aria-hidden="true" />
+        {/if}
+      </button>
     </div>
   </div>
 
-  {#if !hasRepository}
+  {#if collapsed}
+    <!-- collapsed -->
+  {:else if !hasRepository}
     <p class="blank-state">Open a repository to browse files.</p>
   {:else if repoFiles.length === 0}
     <p class="blank-state">No files returned.</p>