refactor(web-ui): align theme bootstrap and color governance

[limityan]

Summary

• Generate the desktop startup theme bootstrap manifest from TS builtin theme presets and load that minimal first-paint projection from Rust via include_str!.
• Shrink generated widget theme payload compatibility surface by removing low-risk accent/primary/danger legacy exports while keeping root/runtime compatibility definitions intact.
• Merge three indistinguishable Mermaid fallback color pairs without removing semantic fallback roles.
• Update the theme optimization document and governance baseline to reflect the lower budgets.

Metrics

Metric	Before	After
Generated widget payload vars	326	307
Generated widget payload compatibility aliases	64	45
Generated widget external-only compatibility keys	81	62
Mermaid unique colors	95	92
Mermaid indistinguishable near pairs	3	0
Specialized-domain indistinguishable near pairs	6	3
Total unique color literals	560	557

Risk Review

• Rust still owns only selection/first-paint bootstrap behavior; full theme schema and token semantics stay in TS.
• Root CSS/runtime legacy aliases remain available, so app-internal historical CSS is not removed in this PR.
• Removed widget payload keys are not dynamically assembled by the widget payload reader; the retired names are covered by a negative contract test and canonical accent/error keys remain exported.
• Mermaid changes share actual values but preserve distinct role keys for node hover fill, edge label hover border, muted text, and hover stroke.
• Expected user-visible effect: desktop first paint now uses TS preset values instead of older Rust approximations, reducing startup theme drift.

Verification

• pnpm run generate-startup-theme-bootstrap -- --check
• pnpm run theme:color-audit:test
• pnpm run theme:color-audit -- --top 20 --report-json .tmp-theme-audit-stage3-after.json
• pnpm run theme:visual-contract
• pnpm --dir src/web-ui run test:run
• pnpm run lint:web
• pnpm run type-check:web
• pnpm run build:mobile-web
• cargo check -p bitfun-desktop
• rustfmt --edition 2024 --check src/apps/desktop/src/theme.rs
• git diff --check
• pnpm run build:web