feat(themes): add Arrakis Dark

[HermesSantos]

Theme submission

Theme

• Name: Arrakis Dark
• Id (folder themes/<id>/): arrakis-dark
• Type: dark
• Author (the name to credit, as you spell it): Hermes Santos
• Author URL (https link to your profile or the theme's repo): https://github.com/HermesSantos
• Source / original author (for ports): n/a — original theme

Description

A desert-inspired dark theme: near-black sand backgrounds (#16120D through
#3A2D1F), spice-gold accent (#D4A017), and warm parchment text. Diff colors
stay in the desert range — olive for added, clay red for removed, gold for
modified — with a single cool slate lane (#7C8A8B) to keep the graph readable.

Checklist

• One theme added or changed in this PR
• Files live in themes/<id>/ and id matches the folder name
• id is a unique lowercase kebab-case slug
• themes/<id>/theme.json validates against schema/theme.schema.json
• All fifteen colors present as #RRGGBB
• meta.version is semantic; meta.author is your name (not a login)
• meta.authorUrl, if present, is an https link
• themes/<id>/README.md added (with the preview@2x.png image)
• themes/<id>/preview@2x.png added (node scripts/gen-previews.mjs <id>)
• node scripts/validate.mjs reports no errors
• Original author credited (for ports)

[gitgusilva]

Checked out locally: node scripts/validate.mjs is clean, the preview renders correctly, and the README/registry entries follow the existing pattern (this one fills the empty cell in the Dark table). The palette is cohesive and the single cool graph lane keeps the graph legible against all that gold. Thanks!