ref(billing): Migrate spend notifications form to new form system

[TkDodo]

Migrates the spend notifications threshold management page (/settings/billing/notifications/) from manual useState/useEffect state management with a raw <Select> and legacy FieldGroup to the new TanStack-based form system.

The page had two multi-select fields (subscription consumption thresholds and on-demand consumption thresholds) with Save/Reset buttons in the page header. The migration:

• Replaces manual state management with useScrapsForm and fetchMutation
• Replaces the raw <Select> + styled legacy FieldGroup with field.Select inside field.Layout.Row within a new-system FieldGroup
• Moves Save/Reset buttons from the SettingsPageHeader action slot into the form body, using form.Subscribe for reactive dirty/valid state
• Adds onChange validation with .min(1) on reservedPercent to prevent saving with no thresholds selected
• Migrates data fetching from useApiQuery to apiOptions with useQuery
• Removes the responsive breakpoint logic that duplicated the buttons — they now live in one place inside the form

The NotificationButtons, ThresholdInputs, and GenericConsumptionGroup components are collapsed into a single ThresholdsForm component since the separation was only needed when buttons lived in the header.

refs DE-1190

[TkDodo]

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[linear-code]

DE-1190 migrate Subscription page