fix(formatter-settings): disable Save when settings unchanged (closes #349)

[EVWorth]

Summary

Adds dirty tracking to FormatterSettingsDialog. The Save button starts disabled when the local state matches the persisted settings — preventing a no-op round-trip to localStorage when the dialog is opened-and-closed without edits. A small accent-colored dot appears in the dialog title as an unsaved-changes indicator. Save re-enables after any field change.

Files

• src/components/editor/FormatterSettingsDialog.tsx — adds useMemo shallowEqual diff; disabled={!isDirty} on Save; dirty-dot in title; short-circuit in handleSave.
• src/components/editor/__tests__/FormatterSettingsDialog.test.tsx — 4 new tests covering: disabled-by-default, enabled-after-change (with dirty dot), Save-click-with-no-changes-doesnt-overwrite, existing save+close flow updated to first mutate a setting.

Test plan

• vitest: src/components/editor/__tests__/FormatterSettingsDialog.test.tsx — 13/13 pass
• vitest: full frontend suite — 1527/1527 pass
• tsc --noEmit clean
• eslint clean
• cargo fmt / clippy — no Rust files touched

Closes #349