Migrate from @policyengine/design-system to @policyengine/ui-kit/legacy (#891)

* Migrate from @policyengine/design-system to @policyengine/ui-kit/legacy

Pure import-path rename — uses the /legacy compat shim added in
@policyengine/ui-kit 0.8.0 which mirrors design-system's API exactly.

The legacy --pe-color-* CSS custom properties (previously consumed via
@import "@policyengine/design-system/tokens.css") are not re-exported from
ui-kit/legacy, so the relevant token subset is inlined directly into the
top-level :root block in globals.css. The downstream @theme bridge that
maps `--pe-color-*` to Tailwind color variables is unchanged. JS is just
a comment update in src/components/Header.jsx (tokens were already
inlined locally).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add changelog fragment

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaxGhenis

changelog.d/migrate-design-system.changed.md

@@ -0,0 +1 @@
+Migrated dashboard from `@policyengine/design-system` to `@policyengine/ui-kit/legacy`.