Add zcashd migration guide and live EOL timeline widget

[pacu]

Summary

Expands the Z3 page into a comprehensive, shareable migration guide for everyone still on zcashd (node operators, miners, exchanges, wallet-CLI users), and adds a live EOL timeline widget.

Migration guide (src/content/zcash-z3/index.mdx)

• EOL warning callout with a "which path should I take?" table mapping each situation to its destination:
  • Full node, no indexing → Zebra (now)
  • Indexed data services → Zebra + Zaino (now)
  • Zcashd wallet CLI → Zallet (backend: Zaino or zebrad)
  • Need more time → Zakura + zcashd-compat mode (temporary bridge only)
• Zakura zcashd-compat section grounded in the official docs: sidecar architecture (no inbound P2P to zcashd), extended EOL past NU6.3 activation, and its limitations (Ironwood unsupported, Orchard rejected post-NU6.3, transparent/Sapling unaffected). Explicitly framed as temporary — all users must ultimately migrate to Z3.
• Dedicated miners section (block templates via Zebra, or Zakura's RPC under compat mode — mining RPCs are removed from the sidecar zcashd).
• "Before you migrate" backup guidance (zcashd data / wallet.dat, Zebra chain state) plus a pointer to the community-maintained Zebra snapshots in Resources.
• Testnet section pointing to the Ironwood-supporting faucet at fauzec.com.
• Community-resource disclaimer (not associated with ZF, ZODL, ZingoLabs, Valar Group, or any other org).
• Fixes a few typos in the intro (ZOLD → ZODL, etc.).

Live EOL timeline widget (src/components/ZcashdEolTimeline.jsx)

• Client component rendering the five milestones from the zcashd End-of-Life documentation, with a pulsing "You are here" marker computed from the visitor's clock, a countdown to the next milestone, and passed-milestone progress styling.
• Styled with the site's --zec-* theme variables (dark mode supported); hydration-safe for the static export.
• Embedded on both /zcash-z3 and /zcash-z3/zcashd_cycle.

Test plan

• pnpm build passes (static export)
• Verified widget markup present in exported HTML for both pages
• Visual check of the timeline widget in light/dark mode

🤖 Generated with Claude Code