feat(site) : add color field to sites + frontend submodule update

- Add color VARCHAR(7) column to sites entity
- Migration with IF NOT EXISTS for idempotence
- Update reference config
- Frontend: site color picker, dark mode, card styling improvements

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/Version20260309120000.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260309120000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add color column to sites table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql("ALTER TABLE sites ADD COLUMN IF NOT EXISTS color VARCHAR(7) NOT NULL DEFAULT ''");
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE sites DROP COLUMN IF EXISTS color');
+    }
+}