fix(ui): require save button for provider reorder and extend cache size range

[javi11]

Summary

• Provider reorder now requires Save: Drag-and-drop sorting previously auto-saved to the backend immediately. It now updates local state only and shows the "Unsaved Changes" banner, requiring the user to click Save Changes — consistent with toggling enabled/disabled or editing other provider fields.
• Fix index bug: handleDrop was looking up indices in config.providers but splicing from formData, which could produce wrong results after multiple reorders without saving. Now uses formData indices consistently.
• Remove reordering overlay: The "Reordering…" loading overlay and disabled-drag guards are no longer needed since the save is deferred.
• Extend cache size slider: Max max_size_gb increased from 200 → 1000 GB with updated tick marks (250 / 500 / 750 / 1000).

Test plan

• Drag providers to reorder — "Unsaved Changes" banner appears, no API call fired
• Click Save Changes — order is persisted correctly
• Toggle provider enabled/disabled — still shows unsaved changes and saves on button click
• Discard changes by navigating away and back — order reverts to saved state
• Cache size slider goes up to 1000 GB

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