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/SyncSettingsDialog.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { Cloud, GitBranch, Plus, Save, Trash2, X } from "@lucide/svelte";
   import type { GitRemote, PullStrategy } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   export let remotes: GitRemote[] = [];
   export let remoteBranches: string[] = [];
@@ -76,8 +77,8 @@
         </div>
 
         <div class="sync-fields">
-          <label><span>{de ? "Remote für Sync" : "Remote used for sync"}</span><select bind:value={draftRemote}><option value="">{de ? "Automatisch wählen" : "Choose automatically"}</option>{#each remotes as remote}<option value={remote.name}>{remote.name}</option>{/each}</select><small>{de ? "Ein Remote ist die gespeicherte Verbindung zu einem Server-Repository." : "A remote is a saved connection to a repository on a server."}</small></label>
-          <label><span>{de ? `Upstream für ${currentBranch || "aktuellen Branch"}` : `Upstream for ${currentBranch || "current branch"}`}</span><select bind:value={draftUpstream}><option value="">{de ? "Kein Upstream" : "No upstream"}</option>{#each remoteBranches as branch}<option value={branch}>{branch}</option>{/each}</select><small>{de ? "Der Upstream ist der Remote-Branch, mit dem Pull, Push und Ahead/Behind verglichen werden." : "The upstream is the remote branch used by Pull, Push, and Ahead/Behind."}</small></label>
+          <label><span>{de ? "Remote für Sync" : "Remote used for sync"}</span><SelectMenu value={draftRemote} options={[{ value: "", label: de ? "Automatisch wählen" : "Choose automatically" }, ...remotes.map((remote) => ({ value: remote.name, label: remote.name }))]} onChange={(value) => { draftRemote = value; }} /><small>{de ? "Ein Remote ist die gespeicherte Verbindung zu einem Server-Repository." : "A remote is a saved connection to a repository on a server."}</small></label>
+          <label><span>{de ? `Upstream für ${currentBranch || "aktuellen Branch"}` : `Upstream for ${currentBranch || "current branch"}`}</span><SelectMenu value={draftUpstream} options={[{ value: "", label: de ? "Kein Upstream" : "No upstream" }, ...remoteBranches.map((branch) => ({ value: branch, label: branch }))]} onChange={(value) => { draftUpstream = value; }} /><small>{de ? "Der Upstream ist der Remote-Branch, mit dem Pull, Push und Ahead/Behind verglichen werden." : "The upstream is the remote branch used by Pull, Push, and Ahead/Behind."}</small></label>
         </div>
       </section>