Add `TuiPrefs` struct and `KeybindPrefs` sub-struct to `settings.rs`
to decouple TUI-specific preferences (theme, font_size, keybinds) from
the agent/project `config.toml` so they survive project switches.
- `TuiPrefs::path()` resolves to `~/.deepseek/tui.toml`; honours
`DEEPSEEK_CONFIG_PATH` env-var redirect used by tests and CI.
- `TuiPrefs::load()` falls back to struct defaults when the file is
absent — no error, backwards-compatible.
- `TuiPrefs::save()` creates `~/.deepseek/` if necessary.
- `TuiPrefs::validate()` normalises theme case and rejects unknown
values with a helpful message.
- 8 new unit tests cover defaults, round-trip TOML serde, validation,
absent-file fallback, and the save→load cycle; all pass.
- `cargo check` clean (0 errors, 0 new warnings beyond expected
dead-code on the new public API).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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