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/AiSettingsDialog.svelte

@@ -3,6 +3,7 @@
   import { AlertCircle, Bot, Check, Cpu, Eye, EyeOff, Gauge, Globe, Key, LoaderCircle, Sparkles, X, Zap } from "@lucide/svelte";
   import { credDelete, credLoad, credSave } from "../git";
   import type { AiSettings, CommitAiLocalProfile, CommitAiProvider, LocalModelOption } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   interface Props {
     settings: AiSettings;
@@ -200,11 +201,7 @@
         </div>
         <label class="cred-field">
           <span class="cred-field-label">Model</span>
-          <select bind:value={localModelId}>
-            {#each localModels as option (option.id)}
-              <option value={option.id}>{option.label} — {formatSize(option.approx_size_mb)}</option>
-            {/each}
-          </select>
+          <SelectMenu value={localModelId} options={localModels.map((option) => ({ value: option.id, label: `${option.label} - ${formatSize(option.approx_size_mb)}` }))} onChange={(value) => { localModelId = value; }} />
         </label>
         <div class="cred-token-hint">
           <AlertCircle size={13} aria-hidden="true" />