[Ticket]: Honest unavailable states on every usage panel

### Type
Feature

### Summary
Render an explicit inline status when the usage backend is unreachable or unconfigured.

### Intent
So an operator is never shown a chart that implies zero usage when the truth is unknown.

### Source of truth (links)
- `apps/console/src/app/api/usage/[...path]/route.ts` (already answers 503 by design)

### Current Behavior
The proxy already returns a deliberate 503 when `USAGE_URL` is unset, but no panel renders that state.

### Expected Behavior
Every usage panel distinguishes unavailable from zero, in words.

### Acceptance Criteria
- [ ] A 503 renders an inline status line naming the reason
- [ ] Zero usage and unknown usage are visually distinct
- [ ] Empty states are inline status lines, not centered placards
- [ ] No spinner persists indefinitely on a failed request

### Out of Scope
Retry or caching behaviour.

### Technical Context
House design rule: empty states are inline status lines unless the screen has nothing else to do.

### Risks
This is the failure mode the whole epic exists to avoid. An empty chart presented as data is a wrong answer, not a missing one.

### Test Plan
Component test with the proxy forced to 503; assert the status text renders and no chart is drawn.

### Verification evidence
- [ ] Test added and proven to fail before the change
- [ ] `just all-checks` green
- [ ] Every new or changed `src` file is <= 200 LoC

### Human accountable owner
@stephane-segning

### AI Usage Declaration
Ticket drafted with AI assistance from a verified repo audit.
Structure and estimates drafted with AI from a verified repository audit (file
line counts, config keys, and dependency state were read from the actual tree,
not assumed). A human owns intent, scope, and the release commitment.

### Human verification completed
- [ ] I can explain this work without referring to the AI-generated text
- [ ] Acceptance criteria are testable
- [ ] Source of truth is a real link, not boilerplate

Governance: https://adorsys-gis.github.io/ai-governance/

Start with apps/console/src/app/api/usage/[...path]/route.ts, then locate the usage-panel components and their component-test entry points. Force the proxy to return 503 and verify each panel renders an inline reason, distinguishes zero from unknown usage, draws no chart, and stops loading; run just all-checks afterward.

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