Add Pay-as-you-go billing & usage settings screen (mock UI)

[reecebrowne]

Adds the Pay-as-you-go (PAYG) billing & usage settings screen as a mock UI, wired into the SaaS config nav. All data is mocked via usePaygMock; no backend yet. PaygSnapshot defines the intended API contract. Demo hooks (?payg=1) allow previewing in the proprietary build and must be removed before merge.

[aikido-pr-checks]

Degraded-state enabledGates excludes OFFSITE_PROCESSING, but the new cap policy says only AI/AUTOMATION pause. This contradiction can yield wrong gate behavior when enabledGates is used.

Suggested change

	state === "DEGRADED"
	? ["CLIENT_SIDE"]
	state === "DEGRADED"
	? ["CLIENT_SIDE", "OFFSITE_PROCESSING"]

Details

✨ AI Reasoning
​​1) The new section models cap-reached behavior in two places: the mocked snapshot state and the gate behavior description.
​2) In degraded mode, the snapshot enables only CLIENT_SIDE, which excludes OFFSITE_PROCESSING.
​3) Elsewhere, the behavior table and user-facing copy state that OFFSITE_PROCESSING should continue working at cap, with only AI and automation pausing.
​4) This creates contradictory assumptions in the same feature and can produce incorrect behavior once enabledGates is consumed.

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