refactor(ui): rename surface tokens to neutral background tokens

Replace --bui-surface-1..5 with --bui-bg-neutral-1..5 for both light
and dark themes. --bui-bg-neutral-1 replaces the deprecated --bui-bg-app,
and --bui-bg-neutral-2..5 extend the scale. The old bare --bui-bg-neutral-1..4
entries are removed from the deprecated section since their names are now
reused; the hover/pressed/disabled variants remain deprecated.

Updates colors.stories.tsx, the ESLint rule and tests, and all migration
documentation and the PR description accordingly.

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

packages/eslint-plugin/src/no-deprecated-bui-tokens.test.ts

@@ -46,7 +46,11 @@ ruleTester.run('no-deprecated-bui-tokens', rule, {
     { code: `const s = 'var(--bui-warning-bg)'` },
     { code: `const s = 'var(--bui-announcement-bg)'` },
     { code: `const s = 'var(--bui-gray-1)'` },
-    { code: `const s = 'var(--bui-surface-1)'` },
+    { code: `const s = 'var(--bui-bg-neutral-1)'` },
+    { code: `const s = 'var(--bui-bg-neutral-2)'` },
+    { code: `const s = 'var(--bui-bg-neutral-3)'` },
+    { code: `const s = 'var(--bui-bg-neutral-4)'` },
+    { code: `const s = 'var(--bui-bg-neutral-5)'` },
     // Unrelated strings — should not warn
     { code: `const s = 'some-other-string'` },
     { code: `const n = 42` },
@@ -83,12 +87,6 @@ ruleTester.run('no-deprecated-bui-tokens', rule, {
       code: `const s = 'var(--bui-bg-app)'`,
       errors: [{ messageId: 'deprecated', data: { token: '--bui-bg-app' } }],
     },
-    {
-      code: `const s = 'var(--bui-bg-neutral-1)'`,
-      errors: [
-        { messageId: 'deprecated', data: { token: '--bui-bg-neutral-1' } },
-      ],
-    },
     {
       code: `const s = 'var(--bui-bg-neutral-2-hover)'`,
       errors: [