feat(ui): introduce semantic color token families and deprecate legacy tokens

Redesigns the BUI color token system in `@backstage/ui`:

- Adds a gray scale (`--bui-gray-1` through `--bui-gray-11`)
- Adds new foreground tokens with explicit hex values (primary, secondary, disabled, positive, negative, warning, announcement)
- Introduces five new semantic color families — Accent, Announcement, Warning, Negative, Positive — each with bg-base, bg-subdued, border, fg-on-base, and fg-on-subdued variants, for both light and dark themes
- Moves all legacy tokens (`--bui-bg-solid-*`, `--bui-bg-neutral-*`, `--bui-bg-danger/warning/success/info`, `--bui-fg-solid`, `--bui-fg-danger/success/info`, `--bui-border-*`, `--bui-shadow`) into a clearly marked `/* Deprecated tokens */` section in both light and dark themes
- Updates the Spotify theme overrides to use the new accent tokens and mark legacy overrides as deprecated
- Rewrites the `Colors` Storybook story to display all token families as a live, theme-aware reference grid
- Adds a new `@backstage/no-deprecated-bui-tokens` ESLint rule to `@backstage/eslint-plugin` that warns when any deprecated BUI token is referenced in JS/TS string literals; the rule is included in the `recommended` config so it applies to all plugin authors automatically

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

packages/eslint-plugin/index.js

@@ -25,6 +25,7 @@ module.exports = {
         '@backstage/no-mixed-plugin-imports': 'warn',
         '@backstage/no-ui-css-imports-in-non-frontend': 'error',
         '@backstage/no-self-package-imports': 'error',
+        '@backstage/no-deprecated-bui-tokens': 'warn',
       },
     },
   },
@@ -36,5 +37,6 @@ module.exports = {
     'no-mixed-plugin-imports': require('./rules/no-mixed-plugin-imports'),
     'no-ui-css-imports-in-non-frontend': require('./rules/no-ui-css-imports-in-non-frontend'),
     'no-self-package-imports': require('./rules/no-self-package-imports'),
+    'no-deprecated-bui-tokens': require('./rules/no-deprecated-bui-tokens'),
   },
 };