fix(settings): user-configurable typography controls for chat and input

[quiet-node]

Summary

• Adds four user-configurable typography tunables under [window] in config.toml: text_base_px, text_line_height, text_letter_spacing_px, and text_font_weight
• Exposes sliders for all four in Settings > Display, with descriptive labels for font weight (Regular/Medium/Semi-bold/Bold)
• Drives the existing CSS variables (--thuki-text-base, --thuki-text-line-height, --thuki-text-letter-spacing, --thuki-text-font-weight) from the resolved config via a new useEffect in App.tsx
• Fixes a regression in patch_document where whole-number JSON saves into missing float fields were persisted as TOML Integer instead of Float, causing the next fractional save to fail with a type mismatch; schema-derived template is now the authoritative type source

Architecture

Follows the documented config checklist exactly: defaults in config/defaults.rs, schema in schema.rs, validation/clamping in loader.rs, frontend projection in ConfigContext, and full test coverage at every layer.

Test plan

• Run bun run test:all:coverage - all 1295 frontend tests + 858 backend tests pass, both coverage gates green
• Run bun run validate-build - lint, format, typecheck, and production build all clean
• Launch app, open Settings > Display, verify all four sliders appear and persist across restarts
• Verify fractional font-size saves (e.g. 15.5 px) succeed after a whole-number save on a fresh config