feat(git): add rebase workflow with UI state handling

This introduces Git rebase commands (start, continue, abort) and
exposes whether a rebase is currently in progress via the status
payload. The UI now blocks committing during an active rebase and
shows a dedicated rebase notice with continue/abort actions.

- Add tauri commands for rebase operations and status detection
- Update frontend to render rebase state and controls
- Adjust conflict/resolution messaging and related UI styling

src/lib/components/StashPanel.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { Archive, Download, Trash2, Upload } from "@lucide/svelte";
+  import { Archive, ChevronDown, ChevronRight, Download, Trash2, Upload } from "@lucide/svelte";
   import type { GitStash } from "../types";
 
   interface Props {
@@ -26,6 +26,7 @@
 
   let message = $state("");
   let includeUntracked = $state(true);
+  let open = $state(false);
 
   function submitPush() {
     onPush(message, includeUntracked);
@@ -37,16 +38,33 @@
   }
 </script>
 
-<section class="panel stash-panel overflow-hidden" aria-label="Git stash">
+<section class="panel stash-panel overflow-hidden" class:collapsed={!open} 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 class="stash-head-actions">
+      <button
+        class="stash-toggle"
+        type="button"
+        onclick={() => { open = !open; }}
+        aria-expanded={open}
+        title={open ? "Collapse stash panel" : "Expand stash panel"}
+      >
+        {#if open}
+          <ChevronDown size={14} aria-hidden="true" />
+        {:else}
+          <ChevronRight size={14} aria-hidden="true" />
+        {/if}
+      </button>
+      <span class="pill pill-count">{stashes.length}</span>
+    </div>
   </div>
 
-  {#if !hasRepository}
+  {#if !open}
+    <!-- collapsed -->
+  {:else if !hasRepository}
     <div class="blank-state">No repository loaded.</div>
   {:else}
     <div class="stash-create">