feat(zcash): add mempool page and navbar mempool link

[MatiasOS]

Description

Adds the Zcash mempool page (/zec/mempool, /tzec/mempool) and shows the navbar mempool link on Zcash networks.

The page gets every pending transaction from a single verbose getrawmempool call and sorts them by fee rate. There are no per-transaction fetches. Pagination is client-side and the page refreshes every 60s, which keeps it within hosted gateway rate limits.

Each row links to /zec/mempool/:txid, which opens the transaction page.

Related Issue

Part of #398. Stacked on #409.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• Performance improvement
• Other (please describe):

Changes Made

• ZcashMempoolPage shows the txid, age, size, fee and fee rate (zat/B) for each pending transaction. On narrow screens, age, size and fee rate are hidden; like the other explorer tables, the table scrolls horizontally, since .dash-table keeps a 600px minimum width.
• Routes and lazy exports for zec|tzec/mempool and zec|tzec/mempool/:filter.
• NavbarLogo now shows the mempool link on Zcash networks too.
• zcash locale strings for the mempool page in all 5 locales.
• A hermetic e2e spec at e2e/tests/shared/mocked/zcash-mempool.spec.ts. The shared RPC mock now also serves a verbose mempool.

Screenshots (if applicable)

Checked locally against mocked RPC data at desktop width and at 400px.

Checklist

• I have run npm run format:fix and npm run lint:fix
• I have run npm run typecheck with no errors
• I have run tests with npm run test:run
• I have tested my changes locally
• I have updated documentation if needed
• My code follows the project's architecture patterns

Additional Notes

• Zebra's verbose mempool entries include size and fee but no vsize or weight, so the fee rate is in zatoshis per byte.
• Tests:
  • npm run test:run: 151 passing.
  • Hermetic Zcash e2e specs: 11 passing.