fix(date-picker): add installable component

[djalmaaraujo]

Summary

• Add an installable RubyUI::DatePicker component that composes Input, Popover, and Calendar.
• Add DatePicker dependencies so rails g ruby_ui:component DatePicker installs the composed components automatically.
• Update the docs example to use DatePicker(id: "date") directly.

Validation

• rails g ruby_ui:component DatePicker --pretend
• bundle exec rake test in gem/ (162 runs, 0 failures)
• bundle exec rake standard in gem/
• pnpm run build:css in docs/
• bin/rails test in docs/ (63 runs, 0 failures)
• bundle exec standardrb in docs/
• Selenium smoke test: DatePicker popover opens and selecting day 15 updates the input to 2026-05-15

Fixes #352