Fix Proceed to Payment dead ends on the subscription lock screen

[posthog]

Problem

• A tenant whose trial ended or was canceled has only one forward path — the Proceed to Payment button on the subscription lock screen — and both of its failure modes end that path with no workaround.
• Silent no-op: the checkout branch ran if (!checkoutProducts.length) return; but never disabled the button, so it looked live and gave no spinner, toast, or redirect. It also ignored hasInvalidCustom, which the update branch already honors.
• Raw Relay error: CheckoutResult has no errors field, so a rejected checkout came back as errors with no data. Relay then rejected with its internal No data returned for operation ... string, which landed verbatim in the toast. The readable server message was only console.error-logged.

Changes

Symptom	Fix
Button looks live but does nothing	Disable it when there is nothing to buy (checkoutProducts.length === 0), matching the update branch
Invalid custom amount ignored in checkout	Toast and stop before starting checkout, same as the update branch
Relay's internal wording shown to users	In the Relay network layer, when the server returns errors and no data, throw the joined GraphQL error message so onError shows it; partial responses (data present) are unchanged

Notes

• The Relay-layer change is global but safe: an errors-and-null-data response already rejected through onError/error boundaries — this only replaces the message text.
• useProductSelection fires on mount and populates checkoutProducts for every non-PAYG selection, so the button enables normally and only disables when the selection is genuinely empty (e.g. pay-as-you-go only).

Test plan

• npm run type-check clean
• npm run lint:biome clean on changed files

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