feat: Port custom themes feature to opentui

[veracioux]

Closes #3633.

Further suggestions (out of scope of this PR)

• Create a zod schema for a theme so we can zod.parse() to validate

Test scenarios:

See demo

Peek.2025-11-01.13-59.mp4

Works with both global config and project-specific config

Also tested with multiple themes defined

Dumped all these into ~/.config/opencode/themes/: https://github.com/scaryrawr/base16-opencode/tree/main/themes. No performance issues observed.

Error scenarios

[veracioux]

@thdxr Just putting this on your radar to avoid duplication of work.