Add configurable subscribe button color style (single/multi)

[mattstratton]

Summary

This PR introduces a new feature for subscribe buttons:

• Adds a button_style param (default: single) to control subscribe button color style:
  • single: All buttons use Castanet primary brand color (with hover/active/dark variants)
  • multi: Each button uses its platform color (legacy behavior)
• Adds a "lift" hover effect to all subscribe buttons in both styles
• Ensures subscribe buttons render correctly in both main content and sidebar contexts
• Fixes sidebar integration so all sidebar blocks stack vertically in a single column
• Restores correct row layout: episodes on the left, sidebar on the right

Testing Steps

• Test both button_style = "single" and button_style = "multi" in params.toml
• Confirm subscribe buttons look correct in both main content and sidebar
• Confirm sidebar and row layout display as intended

Context

• Addresses the original feature request for configurable subscribe button color and improved UX
• Fixes all layout issues introduced during feature development

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