NVIEW_VISUAL_MODE/NVIEW_IMAGE_PROTOCOL bypass config package, not settable via YAML

`NVIEW_VISUAL_MODE` and `NVIEW_IMAGE_PROTOCOL` bypass the config package
and cannot be set via YAML.

`style.go`'s `currentTerminalVisuals`/`terminalVisualsFromEnv` read these two
variables directly with `os.Getenv`, not through `internal/config`. Neither
field exists in `internal/config/config.go`.

Effects:

- The two variables cannot be set through the YAML config file, even though
`README.md`'s "Configuration (env)" section lists them alongside vars that
do support YAML (`NODE_NAME`, `CARDANO_NETWORK`, `PROM_HOST`, etc.),
implying parity that does not exist.
- Every other setting in the codebase is read exclusively through
`config.GetConfig()` per the project's own documented architecture; these
two are the only exception.

## Acceptance criteria

- `NVIEW_VISUAL_MODE` and `NVIEW_IMAGE_PROTOCOL` are added to
`internal/config` (struct fields + envconfig tags) and settable via YAML,
or the README is corrected to state they are env-only and why.
- `style.go` reads the resolved values from `config.GetConfig()` instead of
calling `os.Getenv` directly.

Start by reading internal/config/config.go and style.go, especially currentTerminalVisuals and terminalVisualsFromEnv, then compare the README.md Configuration (env) section with the existing configuration pattern. Verify how YAML-backed settings are resolved and whether these two variables should join that path or remain environment-only. Done means the chosen behavior is implemented consistently and the README and configuration code no longer contradict each other.

Written by the indexing model from the issue text.