Migrate ProgressSpinner to the Signals API (S) #2051

Part of #1971. Read the epic and packages/optimus-ui/.agents/skills/angular-signal-migration/SKILL.md in full before starting, and comment here to claim the issue.

Scope: packages/optimus-ui/src/progressspinner/ — 5 decorator input(s), 1 component class(es), ~123 lines. Size: S.

Also in this PR / notes:

• Remove the deprecated styleClass input of ProgressSpinner (host class comes from the native class attribute) and extend the styleclass-to-class migration: add the selector(s) to MIGRATED_SELECTORS in packages/optimus-ui/schematics/migrations/styleclass-to-class/migration.ts, list them in migrations.json, add spec cases in schematics/test/styleclass-to-class.spec.ts. Then rewrite styleClass/[styleClass] usages in apps/docs.

Verify locally (only this scope, not the whole suite):

• pnpm --filter @openng/optimus-ui test:unit -- --include progressspinner/progressspinner.test.ts
• pnpm --filter @openng/optimus-ui test:schematics (you touched a migration)
• pnpm exec prettier --check <changed files> and pnpm --filter @openng/optimus-ui build

PR: base next, title refactor(optimus-ui): progressspinner component migration to Signals API, link this issue with Closes #<this issue>.