fix(ui): adopt Source Serif 4 for AI prose reading register

[quiet-node]

Summary

• AI prose body now renders in Source Serif 4 (warm cream #ede4d3, 16px / 1.65) instead of Inter
• Self-hosted via @fontsource/source-serif-4 (Latin subset, regular + italic, no runtime network call)
• Scoped to .markdown-body; UI chrome (askbar, buttons, tabs, search trace headers) stays Inter; user bubble (orange gradient) unchanged
• Bumps @tauri-apps/api to ^2.11.0 to match the Rust crate (pre-existing drift surfaced by install)

Test plan

• `bun run test:all:coverage` — Vitest 1166 + cargo 725 pass with 100% line/function/branch coverage
• `bun run validate-build` — clean lint + format + typecheck + build + bundle
• `bun run dev` — open the app, ask the LLM something with a 3-paragraph reply, confirm prose renders in Source Serif 4 and no UI chrome (askbar, buttons, search trace) regresses
• User bubble still renders in Inter with the orange gradient
• Code blocks inside replies still render in monospace