feat(desktop): add conversation typography controls

[tonyfettes]

Summary

• use the sans-serif stack for ordinary app UI while keeping code, editor, viewer, and terminal surfaces monospace
• add an A typography panel with live conversation controls and the reviewed 150% line height and -0.02em letter spacing defaults
• make conversation sizing inherit the global Small/Medium/Large setting until a Typography Lab override is selected
• align transcript summary labels and sidebar/conversation typography tokens

Testing

• moon -C desktop test frontend --target js (423 passed)
• moon -C desktop test internal/auth --target native (12 passed; host-permitted rerun)
• moon -C desktop test package/internal/packaging --target native (15 passed)
• just test (native 3183 passed, JS 2976 passed, cram 27 passed)
• just build
• moon info
• moon fmt
• git diff --check

Known baseline

• just check reaches 0 warnings and 54 errors because current origin/main has existing implicit_impl_as_method warnings that --deny-warn promotes to errors; the non-strict frontend JS check completes with 0 errors.