feat: make graphite the default theme

[benvinegar]

Summary

• make Graphite the default dark theme fallback
• reorder the theme list so Graphite appears first in the theme menu and theme cycling order
• update helper coverage and README examples to match the new default

Verification

• bun run typecheck
• bun test test/ui-lib.test.ts test/app-interactions.test.tsx test/ui-components.test.tsx
• bun run test:tty-smoke
• bun run check:pack
• real tmux source smoke run

Notes

• bun test on fresh main was flaky in test/session-cli.test.ts, so this PR sticks to the targeted theme-related verification plus TTY smoke.