feat(admin): add Package Registry settings UI section

Adds a registry URL field to Admin > Settings so admins can configure
an external package registry without touching env vars or the API directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/js/sw/surfaces/admin/settings.js

@@ -36,6 +36,7 @@ export default function SettingsSection() {
                 message_variant: cfg_.message?.variant || 'info',
                 footer_enabled: !!cfg_.footer?.enabled,
                 footer_text: cfg_.footer?.text || '',
+                package_registry_url: cfg_.package_registry?.url || '',
             });
             setVault(v);
         } catch (e) { sw.toast(e.message, 'error'); }
@@ -78,6 +79,9 @@ export default function SettingsSection() {
                 text: cfg.footer_text,
             }});
 
+            // Package Registry
+            await sw.api.admin.settings.update('package_registry', { value: { url: cfg.package_registry_url } });
+
             sw.toast('Settings saved', 'success');
         } catch (e) { sw.toast(e.message, 'error'); }
         finally { setSaving(false); }
@@ -166,6 +170,13 @@ export default function SettingsSection() {
                 }
             </div>
 
+            <div class="settings-section"><h3>Package Registry</h3>
+                <div class="form-group"><label>Registry URL</label>
+                    <input value=${cfg.package_registry_url} onInput=${e => set('package_registry_url', e.target.value)} placeholder="https://registry.example.com/registry.json" />
+                </div>
+                <span class="text-muted" style="font-size:12px;">URL to a JSON registry index. Enables browsing and one-click install under Packages > Registry.</span>
+            </div>
+
             <div class="settings-section"><h3>Email</h3>
                 <button class="btn-small" onClick=${testEmail}>Send Test Email</button>
             </div>