Chat font size setting

[Yoonwoo-Ha]

Summary

Terminal text has a size setting, but chat text didn't. This PR adds Settings → Chat → Chat font size, a − / + stepper from 11 to 24px that works like the terminal one.

• Until you choose a size, chat text follows the density setting: 14px for comfortable, 13px for compact. A chosen size is saved on the device along with the other settings.
• The chat view multiplies the text-size tokens by chosen / density size (--chat-scale), and the transcript uses the scaled tokens under the same names. Messages, the user's bubbles, code blocks and prompt cards all grow together.
• The rest of the UI keeps its size. So does the composer's text: it stays at 16px, because iOS zooms the page when you type into a smaller input.

4 files changed, including a new settings.test.ts. This PR doesn't depend on #5, #6 or #7. The only overlap is with #5, which removes scroll-behavior: smooth from the same .chat-view rule, so whichever PR merges second needs a one-line rebase.

Testing

• bun run typecheck passes and bun test passes 257/257. On main without this PR it's 256; the extra one is the new settings test.
• Headless Chrome against a test server, raising the size from 14px to 18px:
  • chat body text went from 14 to 18px
  • code went from 12 to 15.4px, and the user's bubble scaled by the same ratio
  • the sidebar and the composer didn't change
  • the size was still 18px after a reload

🤖 Generated with Claude Code