refactor(theme): memoize rendering

[AnnieIj]

Summary

Memoize derived theme data and row rendering to avoid re-renders on unrelated state changes, improving responsiveness on larger data sets.

Changes

src/lib/preferences.tsx

• Wrap formatAmount in useCallback keyed on the amountFormat primitive, not the entire preferences object — avoids recreating the function when unrelated fields (theme, toastDensity, quietMode) change
• Wrap updatePreference in useCallback with zero dependencies for a stable reference
• Wrap the PreferencesContext.Provider value in useMemo so consumers only re-render when the actual dependencies change
• Extract module-level FALLBACK_UPDATE and FALLBACK_FORMAT constants so usePreferences() outside a provider returns stable references

src/components/settings/SettingsPanel.tsx

• Wrap the component with React.memo to prevent re-renders from unrelated parent state
• Move theme/currency/density option arrays to module-level constants to avoid per-render allocations

src/components/settings/SettingsTrigger.tsx

• Wrap handleClose in useCallback so React.memo on SettingsPanel is effective

src/lib/__tests__/preferences.test.tsx

• Move localStorage.clear() to top-level beforeEach to prevent cross-test state leakage
• Add memoization reference-stability tests covering: stable updatePreference, stable formatAmount when non-amountFormat prefs change, formatAmount reference changes when amountFormat changes, filter/state changes propagate correctly

Test Results

Test Suites: 20 passed, 20 total
Tests:       133 passed, 133 total
Snapshots:   1 passed, 1 total

• ESLint: 0 errors, 0 warnings
• TypeScript (tsc --noEmit): 0 errors
• Build (npm run build): Success

Behaviour

No behavioural changes — all existing tests pass without modification. The memoization only affects internal reference stability, not output.

Closes #564

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[mikewheeleer]

well done @AnnieIj, this reads well. merging in 🌟