feat(quota): resolve coding-plan usage windows via the monitor plane - #57

Mirror the official desktop panel's two-plane split in the quota snapshot:

- credits plane (existing): zcode-plan billing/balance + preview with the
  plan JWT — start-plan/trial credit buckets;
- coding plane (new): GET {api.z.ai|open.bigmodel.cn}/api/monitor/usage/
  quota/limit with the raw coding-plan API key in `authorization` (bundle
  wK/Z_ + md) — data.level + data.limits[] windowed usage rows, surfaced
  as a Coding group in the TUI card and in the `quota` CLI / GET /quota.

Behavior fixes from the PR #56 audit:

- a credential without a plan JWT no longer kills the snapshot — credits
  plane degrades to a noted error while the coding plane still resolves
  (the card no longer tells logged-in users they are not logged in);
- coding rows render `remaining` only: live data shows upstream `number`
  is not a comparable total (TIME_LIMIT remaining=3894 / number=1), and
  the official panel likewise never renders X/Y for these windows;
- envelope success semantics mirror the bundle $_ (success !== false and
  code absent/0/200); monitor failures degrade to codingPlan=null the way
  the official fetchQuota().catch(() => null) does;
- non-token unitType on balance rows is now visible instead of dropped;
- empty-everything state reworded ("no quota entries reported by
  upstream").

@see _reverse/NOTEPAD.md "coding-plan 用量平面"