[PREMIUM-002] Add premium profile customization

[chev0004]

Outline

Summary

Makes premium profile customization real: the card style, custom gradient, and accent pickers already in the editor now persist to the database, are gated server-side by the profile.cardThemes entitlement, and render on discovery and public profile cards.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (Database):
    • Add card_color, custom_gradient_from, custom_gradient_to, and accent_override columns to profiles.
  • (Server):
    • Profile save persists the card theme selection; free users are limited to the free colors and premium theme, custom gradient, and accent values are stripped for them.
  • (Discovery):
    • Profile queries resolve the stored theme into a cardTheme for cards, honoring the owner's current entitlement so lapsed subscribers fall back to the free look.
  • (Profile editor):
    • Saved theme, gradient, and accent load back into the editor's pickers and live preview.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang] (discovery cards render saved premium themes)
• /[lang]/u/[id] (public profile card renders saved theme)
• /[lang]/profile (editor loads saved customization)

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

Before (Screenshot)

After (Screenshot)

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Not Doing

• Banner image uploads: customization ships as colors, gradients, and accents; image storage is left for a follow-up decision.

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Other (Remarks)

Builds on #90 (and #89) and includes their commits until they merge; merge those first and this diff reduces to the customization work.

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Checklists

• Code is well-documented (comments, JSDoc, etc.).
• Existing comments were updated as needed.
• No out-of-scope changes are included.
• Write TODO comments where future work is required.
• Removed unnecessary debug code (e.g., console.log, debugger).
• Self-reviewed and tested locally.