portal: budget notice -> generic low-balance wording, $2 floor, username, once per crossing

Wording no longer names an allowance or a credit pack: it says balance, so it is true whether the member is short on allowance, on purchased credit, or both. Trigger is an absolute $2 floor on remaining budget rather than a percentage, so it means the same at any budget size. Recipient greeting uses members.slug (the username) instead of the email local part, which produced 'Hi n,'. The once-only latch now clears when the balance rises back above the floor, so it fires once per crossing rather than once per budget cycle. BUDGET_NOTICES stays off.

CloudronManifest.json

@@ -31,5 +31,5 @@
  "manifestVersion": 2,
  "website": "https://inference.coop",
  "contactEmail": "info@inference.coop",
- "dockerImage": "registry.inference.coop/co-op/member-portal:20261031-87e7c5b"
+ "dockerImage": "registry.inference.coop/co-op/member-portal:49e79aa"
 }