Migrate the default dark mode theme to shadcn/ui's "New York" style, aligning with shadcn's updated defaults.

Reference: https://ui.shadcn.com/docs/tailwind-v4#:~:text=We%27re%20deprecating%20the%20default%20style.%20New%20proj
ects%20will%20use%20new%2Dyork.

Theme Changes (Dark Mode):

• Updated surface colors for better visual hierarchy:
• --card and --popover: oklch(0.145 0 0) → oklch(0.205 0 0) (slightly lighter)
• --primary: oklch(0.985 0 0) → oklch(0.922 0 0) (softer white)
• Updated border/input styling for transparency-based borders:
• --border: solid color → oklch(1 0 0 / 10%) (white with 10% opacity)
• --input: solid color → oklch(1 0 0 / 15%) (white with 15% opacity)
• --ring: oklch(0.439 0 0) → oklch(0.556 0 0) (lighter focus ring)
• Same updates for --sidebar-border and --sidebar-ring
• Updated destructive color: oklch(0.396 0.141 25.723) → oklch(0.704 0.191 22.216) (more vibrant in dark mode)
• Updated wrap_with_directive for Tailwind 4 compatibility: CSS variables are no longer wrapped in @layer base

Copy Code Modal Improvements:

• Updated CSS output to match shadcn.com/themes standard for TailwindCSS 4
• Added filtering to exclude ruby_ui specific variables (warning, success and their foreground variants) from
  copy output
• Increased dialog width with responsive boundaries (sm:max-w-lg lg:max-w-xl)
• Added TailwindCSS 4 compatibility note in modal description

Accessibility Fixes (Homepage):

• Fixed heading hierarchy for proper document structure:
• Logo: Changed from h4 to h2
• HomeView cards: Changed from h3 to h2
• Updated banner subtitle styling: text-foreground opacity-30 → text-primary/70 for better contrast

Context

This update follows shadcn/ui's transition to the "New York" theme as their new default. The New York style features:

• Softer, more subtle color palette in dark mode
• Transparency-based borders for a modern look
• Slightly elevated card/popover surfaces for better visual separation

Test Plan

• Verify dark mode colors render correctly across all components
• Confirm accessibility improvements with heading structure
• Test border/input transparency styling in various contexts
• Test "Copy Code" modal outputs valid TailwindCSS 4 compatible CSS
• Visual regression testing on homepage and component documentation