[P2] Make dashboard metrics match backend data

[samithreddychinni]

Problem

The dashboard shows numbers and labels that do not match the backend data closely enough.

One concrete bug is in dashboard/src/pages/Overview/index.jsx:74: it reads mismatchStats?.total_7d, while internal/adminapi/mismatches.go:75 returns last_7_days. That card will show 0 even when the API has data.

There are also labels that overstate what the code measures. False signals caught is based on a simple webhook/final-state contradiction query. Gateway degraded in dashboard/src/pages/Overview/index.jsx:80 is inferred from exhausted merchant deliveries or rejected webhooks. Those are useful signals, but they do not prove the gateway is degraded.

Scope

• Fix the mismatch stats field name used by the overview page.
• Rename dashboard labels so they describe the underlying query exactly.
• Replace False signals caught with a label tied to the mismatch query, for example Webhook contradictions.
• Replace Gateway degraded with a local system or action banner based on the actual condition, such as callback delivery failures or webhook rejection rate.
• Remove unsupported gateways from mock data in dashboard/src/lib/mockData.js:23 unless those gateways are backed by real adapters.
• Make every overview card traceable to one backend query or one documented frontend calculation.

Acceptance criteria

• The mismatch count card uses last_7_days and shows nonzero values when the API returns them.
• Overview labels do not claim gateway outage or false-signal prevention unless the backend query proves that exact condition.
• Mock transaction gateways are PayU-only until other gateway adapters exist.
• Dashboard tests or component-level checks cover the mismatch stats mapping.
• Admin API query tests still cover the mismatch count endpoint.