feat(web): Add Kanagawa theme

[vladchatware]

Summary

Adds a built-in Kanagawa desktop theme (light + dark variants) to OpenCode’s default theme registry.

Changes

• add packages/ui/src/theme/themes/kanagawa.json
• register/export Kanagawa in packages/ui/src/theme/default-themes.ts

Verification

• ran the app and confirmed Kanagawa appears in theme selection
• switched between light and dark modes and verified colors are applied correctly in both

Screenshots

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[adamdotdevin]

@vladchatware screenshots for light and dark please!

[vladchatware]

@adamdotdevin updated, please take a look.

[vladchatware]

@adamdotdevin there is Kanagawa in tui, but none in the app.