feat: add deterministic gradient fallback for creator avatars

[extolkom]

Closes #405

What

Added a deterministic gradient fallback for creator profile covers when no cover image is set.

Why

Profiles without a cover image showed a plain flat background that felt unfinished. A gradient derived from the creator handle or ID gives every profile a unique, polished appearance with zero configuration.

Changes

• Added a gradient helper that hashes a creator handle or ID into a deterministic CSS linear-gradient
• Applied the gradient as the cover background when no image is present
• Existing image rendering path is unchanged
• Color ranges are constrained to ensure WCAG AA contrast for text rendered over the gradient

Testing

• Same handle/ID always produces the same gradient
• Profiles with an existing cover image are unaffected
• Existing tests pass

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