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/ComparePanel.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { ArrowRight, GitCompare, LoaderCircle } from "@lucide/svelte";
   import type { GitCommit, GitCommitComparison } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   interface Props {
     commits: GitCommit[];
@@ -40,6 +41,8 @@
     event.preventDefault();
     onCompare();
   }
+
+  let commitOptions = $derived(commits.map((item) => ({ value: item.hash, label: commitOptionLabel(item), group: "Commits" })));
 </script>
 
 <section class="compare-panel panel" aria-label="Compare commits">
@@ -61,32 +64,14 @@
     <form class="compare-form" onsubmit={handleSubmit}>
       <label class="compare-field">
         <span>From (older)</span>
-        <select
-          value={compareFrom}
-          onchange={(e) => onCompareFromChange((e.target as HTMLSelectElement).value)}
-          disabled={isBusy}
-        >
-          <option value="" disabled>Select a commit</option>
-          {#each commits as item (item.hash)}
-            <option value={item.hash}>{commitOptionLabel(item)}</option>
-          {/each}
-        </select>
+        <SelectMenu value={compareFrom} options={commitOptions} placeholder="Select a commit" showSelectedGroup disabled={isBusy} onChange={onCompareFromChange} />
       </label>
 
       <ArrowRight class="compare-arrow" size={18} aria-hidden="true" />
 
       <label class="compare-field">
         <span>To (newer)</span>
-        <select
-          value={compareTo}
-          onchange={(e) => onCompareToChange((e.target as HTMLSelectElement).value)}
-          disabled={isBusy}
-        >
-          <option value="" disabled>Select a commit</option>
-          {#each commits as item (item.hash)}
-            <option value={item.hash}>{commitOptionLabel(item)}</option>
-          {/each}
-        </select>
+        <SelectMenu value={compareTo} options={commitOptions} placeholder="Select a commit" showSelectedGroup disabled={isBusy} onChange={onCompareToChange} />
       </label>
 
       <button class="btn-primary" type="submit" disabled={!canCompare}>