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/CommitNoteDialog.svelte

@@ -11,6 +11,7 @@
     X,
   } from "@lucide/svelte";
   import type { AppLanguage, GitCommit, GitRemote } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   interface Props {
     commit: GitCommit;
@@ -196,11 +197,7 @@
           <div class="commit-note-sync-controls">
             <label>
               <span>{text.remote}</span>
-              <select bind:value={selectedRemote} disabled={isBusy}>
-                {#each remotes as remote (remote.name)}
-                  <option value={remote.name}>{remote.name}</option>
-                {/each}
-              </select>
+              <SelectMenu value={selectedRemote} options={remotes.map((remote) => ({ value: remote.name, label: remote.name }))} disabled={isBusy} onChange={(value) => { selectedRemote = value; }} />
             </label>
             <button type="button" onclick={() => onFetch(selectedRemote)} disabled={isBusy || !selectedRemote || hasChanges}>
               {#if isBusy}<LoaderCircle class="spin" size={15} aria-hidden="true" />{:else}<Download size={15} aria-hidden="true" />{/if}