fix: stop SettingsPage crashing on cold load

[Mng-dev-ai]

Summary

• On a hard refresh of the Settings page, there's exactly one render where the query's settings have resolved (so loading is false) but the useEffect that syncs localSettings hasn't fired yet — leaving localSettings === null.
• That render crashed two ways: the post-gate invariant throw'd, and hasUnsavedChanges dereferenced the null localSettings via buildChangedPayload (null is not an object).
• Guard hasUnsavedChanges against null localSettings, and replace the post-gate throw with the loading spinner so the page waits one render for the sync effect.
• Consolidate the now-redundant if (loading) branch into the single !settings || !localSettings spinner gate (isLoading is only true when there's no data yet), dropping the unused loading binding.
• Kept the useEffect sync rather than a render-time ref check — localSettings is settings-form state (a copy of server data the user edits independently), which CLAUDE.md designates as the correct case for effect-based syncing.

Test plan

• Hard-refresh on /settings (each tab) — page loads without crashing
• Navigate to /settings from a warm cache — no spinner flash, content renders immediately
• Edit a field, confirm the unsaved-changes banner appears and Save/Discard work
• Simulate a settings fetch failure — "Failed to load settings" still shows