feat(ui): searchable model picker with provider group headers — v0.50.86 (PR #659 by @mmartial)

- Live search input in model dropdown (filter by name or ID)
- Provider group headers preserved in filtered view
- Clear button, Escape-to-close, No models found empty state
- i18n EN/ES/zh-CN strings
- CSS uses var(--accent) consistent with current theme system
- zh-CN double-escape fix included
- Provider headers regression fix included
- 1423 tests pass

Co-authored-by: mmartial <mmartial@users.noreply.github.com>

static/index.html

@@ -592,7 +592,7 @@
                 <div class="settings-section-title">System</div>
                 <div class="settings-section-meta">Instance version and access controls.</div>
               </div>
-              <span class="settings-version-badge">v0.50.85</span>
+              <span class="settings-version-badge">v0.50.86</span>
             </div>
             <div class="settings-field" style="border-top:1px solid var(--border);padding-top:12px;margin-top:8px">
               <label for="settingsPassword" data-i18n="settings_label_password">Access Password</label>