Adopt the Fleet code-health baseline

[sarthakagrawal927]

Summary

• add one measurable no-regression gate for the web, Astro landing, and native SwiftUI surfaces
• run the complete web gate on Linux and native format/tests/build/coverage gate on macOS CI
• remove 14 disconnected legacy files, 11 dead server actions, and unnecessary public exports; Knip is now fully clean
• fix the hidden Astro 5 / Vite 8 tooling mismatch by pinning the landing workspace to Astro-compatible Vite 6
• document exact remaining debt and ratchets in Reduce accepted code-health debt #89

No runtime production dependency, product behavior, migration, deployment, production configuration, or secret changed.

Current evidence

• Format/Biome/TypeScript: clean
• Astro: 11 files, 0 errors, 0 warnings, 0 hints
• Web tests: 482 passing; 99.8% scoped lines, 94.69% branches, 100% functions
• Unused: 0 files, 0 exports, 0 types, 0 dependencies, 0 unresolved
• Complexity: 85 accepted violations; max CCN 46, max length 741, max parameters 12
• Duplication: 52 groups, 945 lines, 1.6015863331299574%
• Cycles: 0 TypeScript cycles; Xcode resolves the Swift target graph
• Dependencies: 0 critical, 21 exact accepted high advisory IDs, 0 unexpected
• Suppressions: 11 accepted markers
• Native: 9 unit tests and 4 UI tests passing; Release build passing; 2,649/4,188 production lines (63.2521%)
• Production web build: 706 static pages generated
• Documentation build: 29 pages generated

Validation

• pnpm quality
• pnpm quality:native
• pnpm build
• pnpm docs:build
• pnpm --filter significanthobbies-landing-astro build
• workflow YAML parse

Closes #88