Take design packages back out of plugins

A plugin can no longer replace the site palette. "Designpaket übernehmen"
wrote the plugin's colours, fonts and buttons into the website for good:
no confirmation, no way back, right next to a switch that is safe to flip.
Colours belong to the design screen, where they have a preview and one-step
undo, so the feature is gone rather than guarded (ADR 105).

Removed: PluginRegistry::themePackage() and applyTheme(), "theme" in the
manifest and in the plugin structure, the admin_plugin_theme route and the
Theme dependency of PluginAdminController, the button and the design-package
line on the "Plugins" screen, and plugins/nachtblau. A manifest now needs at
least one block, one route or one preset.

The test says the opposite of what it used to: switching a plugin on and off
leaves both the design and the content alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/EditorSecurityTest.php

@@ -161,7 +161,7 @@ final class EditorSecurityTest extends WebTestCase
             ['publisher', 'GET', '/admin/aufgaben'], ['publisher', 'POST', '/admin/aufgaben/ausfuehren'],
             ['publisher', 'GET', '/admin/benutzer'], ['publisher', 'POST', '/admin/benutzer/neu'],
             // Ein Plugin bringt Code auf die Website: Administrator.
-            ['publisher', 'GET', '/admin/plugins'], ['publisher', 'POST', '/admin/plugins/vergleich/an'], ['publisher', 'POST', '/admin/plugins/vergleich/einstellungen'], ['publisher', 'POST', '/admin/plugins/nachtblau/design'],
+            ['publisher', 'GET', '/admin/plugins'], ['publisher', 'POST', '/admin/plugins/vergleich/an'], ['publisher', 'POST', '/admin/plugins/vergleich/einstellungen'],
             ['publisher', 'POST', '/admin/pages/p1/delete'],
             // Alles, was sofort auf der Website steht: Veroeffentlicher.
             ['editor', 'GET', '/admin/settings'], ['editor', 'POST', '/admin/settings/redirects'], ['editor', 'POST', '/admin/settings/rechtliches'], ['editor', 'POST', '/admin/settings/not-found'],