Implemented the dynamic theme manager and merged the supplied themes.

Included

* Standardised all six theme.json files to manifest version 2:
    * admin
    * common
    * corporate
    * saasland
    * swiss
    * welcome
* Every manifest now uses the same fields and structure.
* Separate active frontend and backend themes.
* One-click theme activation.
* Theme preview in a new browser tab without activating it.
* Theme cards showing:
    * Preview
    * Name
    * Version
    * Author
    * Description
    * Theme type
    * Active status
* Dynamic theme-specific settings.
* Support for:
    * Text
    * URL
    * Colour
    * Textarea
    * Checkbox fields
    * Variants
    * Appearance
    * Accent palettes
* Theme ZIP installation.
* Theme ZIP export.
* Asset optimisation.
* Accessibility validation.
* Safe ZIP path validation.
* Theme inheritance.
* Active themes cannot be uninstalled.
* Parent themes cannot be uninstalled while child themes depend on them.
* Stylist administration routes now require an authenticated, verified user.
* Manifest validation now restricts theme types to:
    * frontend
    * backend
    * common
* Added activation tests for independent frontend and backend theme selection.

The supplied corporate theme was incomplete, so I added the required backend entry view and placeholder asset files while preserving its supplied layout.

Modules/Stylist/Theme/ManifestValidator.php

@@ -29,6 +29,9 @@ final class ManifestValidator
         if (isset($manifest['version']) && (! is_string($manifest['version']) || ! preg_match('/^\d+\.\d+\.\d+(?:[-+][0-9A-Za-z.-]+)?$/', $manifest['version']))) {
             $errors[] = 'The version must use semantic versioning, for example 1.2.0.';
         }
+        if (isset($manifest['type']) && ! in_array($manifest['type'], ['frontend', 'backend', 'common'], true)) {
+            $errors[] = 'The type must be frontend, backend, or common.';
+        }
         $parent = $manifest['extends'] ?? $manifest['parent'] ?? null;
         if ($parent !== null && (! is_string($parent) || ! preg_match('/^[a-z0-9][a-z0-9_-]*$/', $parent))) {
             $errors[] = 'The extends/parent value must be a valid theme key.';