feat: give the RustMail palette warm iron neutrals and an oxide accent

[xsmyile]

RustMail was Ember with another gradient; this makes it the one warm palette, with an amber-to-oxide glow and selection.

[pullfrog]

✅ No new issues found.

Reviewed changes — Reviewed the RustMail palette update in ui/src/index.css and its shared theme and component consumers.

• Warm neutrals and oxide accents: Updates the RustMail zinc and orange color scales.
• Light and dark gradients: Adjusts the amber-to-oxide canvas glow and inbox selection backgrounds.

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