Add changesets for CSS exports support

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

.changeset/css-exports-support.md

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+---
+'@backstage/cli': minor
+---
+
+Added support for CSS exports in package builds. When a package declares a CSS file in its `exports` field, the CLI will now automatically bundle it during `backstage-cli package build` using lightningcss, and rewrite the export path from `src/` to `dist/` at publish time. This allows packages to include standalone CSS files without custom build scripts.

.changeset/ui-standard-build.md

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+---
+'@backstage/ui': patch
+---
+
+Migrated to use the standard `backstage-cli package build` for CSS bundling instead of a custom build script.