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/GlobalSearchDialog.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+  import SelectMenu from "./SelectMenu.svelte";
   import {
     CalendarDays,
     FileCode,
@@ -193,12 +194,7 @@
 
             <label class="search-limit">
               <span>Results</span>
-              <select bind:value={limit} disabled={!hasRepository || isBusy || isSearching}>
-                <option value={100}>100</option>
-                <option value={250}>250</option>
-                <option value={500}>500</option>
-                <option value={1000}>1000</option>
-              </select>
+              <SelectMenu value={String(limit)} options={[100, 250, 500, 1000].map((value) => ({ value: String(value), label: String(value) }))} disabled={!hasRepository || isBusy || isSearching} onChange={(value) => { limit = Number(value); }} />
             </label>
 
             <button class="btn-primary" type="submit" disabled={!hasRepository || isBusy || isSearching || query.trim().length === 0}>