refactor(settings): seed system prompt body to show in settings panel

[quiet-node]

Summary

• Settings panel was showing the System Prompt as empty because the on-disk
  [prompt] system field was treated as "use built-in" when blank, so the
  textarea had nothing to render. The persona was hidden inside a Rust
  constant the user could not see or edit.
• This change makes the config file the single source of truth: on first
  run the loader seeds [prompt] system with the full built-in body, so
  the textarea always shows the actual prompt the model receives. Editing
  the textarea now edits the prompt in place, no fallback constant
  consulted at runtime.
• The slash-command appendix continues to be auto-composed at load time
  (still never written to disk) so /search etc. keep working.
• Default textarea height bumped (rows 16) and char cap raised (8000 → 32000)
  so the ~17 KB built-in body is readable without manual resizing.
• Tiny gitignore tidy: ignore SearXNG settings.yml.new dump.

Test plan

• bun run test:all:coverage — 100% lines/branches/funcs frontend; cargo llvm-cov passes --fail-under-lines 100
• bun run validate-build — zero warnings, zero errors
• Fresh-install smoke: delete ~/Library/Application Support/com.quietnode.thuki/config.toml, launch app, open Settings → AI → Prompt, confirm textarea is prefilled with the persona body
• Edit textarea, confirm change persists across app relaunch
• Clear textarea entirely, confirm [prompt] system = "" saves cleanly and only the slash-command appendix is sent