fix: surface save-backed General settings first

[atomantic]

Summary

• Reorder General settings so Timezone and Location appear before the instant-apply Interface Theme gallery.
• Preserve the existing theme picker, save actions, and unsaved-change guards.
• Add a regression assertion requiring both headings and their document order.

Test plan

• ./node_modules/.bin/vitest run src/components/settings/GeneralTab.test.jsx
• npm run lint
• npm run build
• Full npm test was attempted; an unrelated accessibility convention test repeatedly probed an unavailable local port 3000.

Closes #6948