feat(git stash): add stash listing and push/apply/pop/drop UI

This change introduces Git stash support end-to-end, including a new
backend command to list stashes and operations to push, apply, pop, and
drop them. The frontend now fetches stashes as part of the repository
bundle and provides a dedicated panel to manage shelved changes.

- Add GitStash model and Tauri commands for stash operations
- Implement StashPanel component and wire it into the app
- Adjust sidebar layout and add stash-specific styling

src/lib/components/StashPanel.svelte

@@ -0,0 +1,116 @@
+<script lang="ts">
+  import { Archive, Download, Trash2, Upload } from "@lucide/svelte";
+  import type { GitStash } from "../types";
+
+  interface Props {
+    stashes: GitStash[];
+    changedCount: number;
+    hasRepository: boolean;
+    isBusy: boolean;
+    onPush: (message: string, includeUntracked: boolean) => void;
+    onApply: (stash: GitStash) => void;
+    onPop: (stash: GitStash) => void;
+    onDrop: (stash: GitStash) => void;
+  }
+
+  let {
+    stashes = [],
+    changedCount = 0,
+    hasRepository = false,
+    isBusy = false,
+    onPush = () => {},
+    onApply = () => {},
+    onPop = () => {},
+    onDrop = () => {},
+  }: Props = $props();
+
+  let message = $state("");
+  let includeUntracked = $state(true);
+
+  function submitPush() {
+    onPush(message, includeUntracked);
+    message = "";
+  }
+
+  function stashTitle(stash: GitStash): string {
+    return stash.message || stash.selector;
+  }
+</script>
+
+<section class="panel stash-panel overflow-hidden" 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>
+    <span class="pill pill-count">{stashes.length}</span>
+  </div>
+
+  {#if !hasRepository}
+    <div class="blank-state">No repository loaded.</div>
+  {:else}
+    <div class="stash-create">
+      <input
+        class="stash-input"
+        type="text"
+        bind:value={message}
+        placeholder="Optional message"
+        disabled={isBusy || changedCount === 0}
+        onkeydown={(event) => {
+          if (event.key === "Enter" && changedCount > 0 && !isBusy) submitPush();
+        }}
+      />
+      <label class="stash-check">
+        <input type="checkbox" bind:checked={includeUntracked} disabled={isBusy || changedCount === 0} />
+        Untracked
+      </label>
+      <button
+        class="btn-sm stash-save-button"
+        type="button"
+        onclick={submitPush}
+        disabled={isBusy || changedCount === 0}
+        title="Save current working tree changes to a stash"
+      >
+        <Archive size={14} aria-hidden="true" />
+        Stash
+      </button>
+    </div>
+
+    {#if stashes.length === 0}
+      <div class="blank-state stash-empty">No stashes saved.</div>
+    {:else}
+      <div class="stash-list">
+        {#each stashes as stash (stash.selector)}
+          <article class="stash-row">
+            <div class="stash-row-main">
+              <strong title={stashTitle(stash)}>{stashTitle(stash)}</strong>
+              <span>
+                {stash.selector}
+                {#if stash.branch}
+                  on {stash.branch}
+                {/if}
+                {#if stash.date}
+                  - {stash.date}
+                {/if}
+              </span>
+            </div>
+            <div class="stash-actions">
+              <button class="btn-sm" type="button" onclick={() => onApply(stash)} disabled={isBusy} title="Apply stash and keep it">
+                <Download size={13} aria-hidden="true" />
+                Apply
+              </button>
+              <button class="btn-sm" type="button" onclick={() => onPop(stash)} disabled={isBusy} title="Apply stash and remove it if successful">
+                <Upload size={13} aria-hidden="true" />
+                Pop
+              </button>
+              <button class="btn-sm danger" type="button" onclick={() => onDrop(stash)} disabled={isBusy} title="Delete stash">
+                <Trash2 size={13} aria-hidden="true" />
+                Drop
+              </button>
+            </div>
+          </article>
+        {/each}
+      </div>
+    {/if}
+  {/if}
+</section>