feat(network): add Zcash network dashboard

[MatiasOS]

Description

Adds the Zcash network dashboard at /zec and /tzec, and makes both networks visible across the app.

The dashboard is designed around the hosted gateway limit of about 5 requests per minute per IP:

• Stats and the tip block load first, in 3 calls. Earlier block summaries then load one at a time.
• Block summaries are cached across refreshes, so each 90s refresh usually costs 3–4 calls.
• The navbar block indicator polls Zcash every 90s. Without this it would use the 12s EVM default, which alone is about 5 calls a minute.

Related Issue

Part of #398. Stacked on #406.

Type of Change

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

Changes Made

• src/config/networks.json: zec and tzec entries, taken from explorer-metadata with slugs added.
• /zec and /tzec routes, with a lazy-loaded page.
• useZcashDashboard plus the dashboard components:
  • Stats: height, difficulty, mempool, and chain supply with the shielded share.
  • A value pools bar covering Transparent, Sprout, Sapling, Orchard, Lockbox and Ironwood.
  • Latest blocks.
  • Latest transactions, with a badge for each transaction's kind.
• i18n:
  • A new zcash namespace and a tooltips.zcash section in all 5 locales.
  • Relative times are localized, instead of reusing the English-only Bitcoin formatter.
• NetworkBlockIndicator: a Zcash branch with a 90s poll.
• useRpcLatencyTest: Zcash endpoints are tested with getblockcount.
• README.md and .claude/rules/commands.md: list the Zcash networks and slugs.
• A hermetic e2e spec at e2e/tests/shared/mocked/zcash-dashboard.spec.ts, with RPC traffic served from mainnet block 3,483,400.

Screenshots (if applicable)

Checked locally against mocked RPC data in dark, light and 400px-wide layouts.

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

• METADATA_VERSION isn't bumped yet. The explorer-metadata publish of @openscan/metadata 1.3.0-alpha.0 failed, because npm requires --tag for prereleases. Until it's published:
  • Zcash RPCs come from the built-in defaults added in feat(adapter): add Zcash data layer #406.
  • The network icon falls back to the generic one.
• The dashboard links to the blocks, block, txs and tx pages. Those pages arrive in the next PRs of the stack.
• The navbar mempool link will ship with the mempool page.
• Tests:
  • npm run test:run: 150 passing.
  • npx playwright test --project=mocked e2e/tests/shared/mocked/zcash-dashboard.spec.ts: 2 passing.