feat(ui): add SelectMenu component and replace native selects

Introduce a reusable SelectMenu component and replace native select
controls across the UI. This centralizes select behavior and styling,
enabling grouped options, placeholders, and a consistent popup
interaction. Add comprehensive CSS and light-theme tweaks, and update
rebase action styling to integrate the new control.

- Add a unified SelectMenu component and wire change handlers.
- Implement .select-menu styles, popup behavior, and theme overrides.
- Replace ad-hoc native selects in dialogs and rebase UI for consistency.

src/lib/components/AiCommitSplitDialog.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { GitCommitHorizontal, LoaderCircle, Sparkles, X } from "@lucide/svelte";
   import type { AiCommitPlan } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   interface Props {
     plan: AiCommitPlan;
@@ -62,9 +63,7 @@
           <div class="split-files">
             {#each group.files as file}
               <div><code>{file}</code>
-                <select value={groupIndex} onchange={(event) => moveFile(file, groupIndex, Number(event.currentTarget.value))} disabled={isApplying}>
-                  {#each draft.groups as _, target}<option value={target}>Commit {target + 1}</option>{/each}
-                </select>
+                <SelectMenu class="split-file-target" value={String(groupIndex)} options={draft.groups.map((_, target) => ({ value: String(target), label: `Commit ${target + 1}` }))} disabled={isApplying} onChange={(value) => moveFile(file, groupIndex, Number(value))} />
               </div>
             {/each}
           </div>
@@ -98,6 +97,7 @@
   .split-files{display:grid;gap:5px}
   .split-files>div{display:flex;align-items:center;gap:10px;padding:6px 8px;border-radius:6px;background:var(--color-surface)}
   code{min-width:0;flex:1;overflow:hidden;text-overflow:ellipsis;color:var(--color-ink-muted);font-size:11px;white-space:nowrap}
-  select{height:28px;border:1px solid var(--color-border-subtle);border-radius:5px;background:var(--color-surface-raised);color:var(--color-ink);font-size:11px}
+  :global(.split-file-target){width:110px;flex:0 0 110px}
+  :global(.split-file-target .select-menu-trigger){height:28px;min-height:28px;font-size:11px}
   .dialog-footer p.invalid{color:#e0a040}
 </style>