fix: protect unsaved General settings edits

[atomantic]

Summary

• track independent saved baselines for timezone and location edits
• show section-specific unsaved status and guard route changes plus browser unloads
• preserve dirty state through failed saves, serialize editable save state, and reset drafts safely on discard
• cover routed navigation, independent saves, load failures, stale StrictMode results, and in-flight save branches

Test plan

• cd client && npm test (824 files, 10,461 tests)
• cd client && npm run lint
• cd client && npm run build

Closes #5480