refactor(theme): retire legacy Rust theme schema

[limityan]

Summary

• retire the legacy Rust top-level GUI theme config schema/provider/default export, keeping only a thin theme.id -> themes.current fallback for old config loads/imports and old call paths
• keep import compatibility raw until manager normalization so old exported files with config.theme.id do not lose their selected theme before deserialization
• update the theme optimization design doc so Rust owns only theme selection persistence/bootstrap projection while TS/web-ui owns the full theme contract
• merge low-risk web preset near colors, regenerate generative UI theme prompt snapshots, and keep installer named dark preview backgrounds after product review confirmed adjacent theme-card identity risk

Color governance impact

Metric	Before	After
web unique colors	345	342
web theme preset unique colors	119	116
web theme preset near pairs	9	5
web specialized near pairs	13	9
installer unique colors	62	62
installer theme preset unique colors	53	53
installer theme preset near pairs	3	3

Compatibility and risk notes

• Legacy top-level theme payloads are normalized on load/import/save by moving only theme.id into themes.current; other old GUI theme fields are intentionally no longer a public extension contract.
• dark and light legacy IDs normalize to bitfun-dark and bitfun-light; existing non-empty themes.current is not overridden.
• The desktop import command now accepts the existing request.configData envelope and forwards raw config JSON to the config manager before GlobalConfig deserialization.
• terminal.theme remains a terminal ANSI/defaults config field and is not treated as the GUI theme contract.
• Product review kept Ink Night secondary and installer named dark primary previews because they affect Monaco line highlight / adjacent theme-card recognition.

Verification

• git diff --check
• pnpm run theme:color-audit:all
• pnpm run theme:color-audit:test
• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run -- startupThemeBootstrap.test.ts themePresetOutput.test.ts
• pnpm --dir src/web-ui run test:run
• pnpm --dir BitFun-Installer run type-check
• cargo test -p bitfun-core legacy_theme -- --nocapture
• cargo test -p bitfun-core generated_theme_prompt -- --nocapture
• cargo check -p bitfun-desktop
• cargo check --workspace

cargo check/cargo test still report existing warnings in bitfun-services-integrations and bitfun-desktop; this PR does not introduce new Rust warnings in the touched config code.