fix(about): pick team avatar variant by grid position

[felixwidjaja]

Summary

• /about/team backdrops now rotate glacier → canyon → pine by card position, and each card shows the headshot variant tuned to its backdrop.
• Variants are resolved per slug from the Strapi media library (teamAvatarVariants.ts, cached under the authors tag), so adding or removing a person no longer shifts the rest of the roster out of rhythm.
• Falls back to the author's pinned avatar when a variant isn't uploaded. The profile modal uses the same result as the card.
• Triggered by the four new bios in Add new team bios to the Datum website #1771 (Viktor, Karthik, Mykola, Andrii), which landed with mismatched backdrops.

Notes

• Department tab filtering hides cards client-side, so the rhythm is only guaranteed in the "Everyone" view.
• /hello still uses pine-forge and is unchanged.

Test plan

• astro check, ESLint, Prettier pass
• Media library has 93 variant files, all parsed by the new helper
• /about/team shows glacier, canyon, pine in order with matching hair tones
• Profile modal backdrop matches the card