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>

src/Entity/Site.php

@@ -66,6 +66,9 @@ class Site
     #[ORM\Column(type: Types::STRING, length: 100, options: ['default' => ''], name: 'contactCity')]
     private string $contactCity = '';
 
+    #[ORM\Column(type: Types::STRING, length: 7, options: ['default' => ''], name: 'color')]
+    private string $color = '';
+
     #[ORM\Column(type: Types::DATETIME_IMMUTABLE, name: 'createdAt')]
     private DateTimeImmutable $createdAt;
 
@@ -164,6 +167,18 @@ class Site
         return $this;
     }
 
+    public function getColor(): string
+    {
+        return $this->color;
+    }
+
+    public function setColor(string $color): static
+    {
+        $this->color = $color;
+
+        return $this;
+    }
+
     /**
      * @return Collection<int, Machine>
      */