Add Sign in with Zcash use case page

[shadowosx]

Adds a "Sign in with Zcash" page to the Use Cases section, covering what the pattern is, how it works, the trade-offs, and the projects exploring it.

Two approaches are documented separately, since they have different requirements and costs:

• Signing a challenge, which is free and instant but depends on wallet support for message signing (ZIP 304 is still a draft)
• Sending a shielded payment with a one time code in the memo, which works with most wallets today but costs a fee and waits for confirmation

The trade-offs section covers both, plus the shared limitations worth being honest about: no account recovery by default, address reuse recreating the tracking problem, no ratified standard so implementations don't interoperate, and the fact that it hides your finances from an app without making you anonymous inside it.

The six projects listed are from the Zcash Login track of Hackathon 3.0. Each description is taken from that project's own README rather than summarised secondhand, and they're framed as experiments rather than finished products.

Notes:

• Follows the format of the existing standalone pages in this folder, so no "Step X of 6" footer and no change to the recommended path in About.md, matching how Keeping Records With Shielded ZEC sits alongside the numbered journey
• All nine icons reuse existing self hosted assets in content-images, no new images needed
• Internal links checked against the current tree, external links return 200
• Companion PR adding the sidebar entry: Add Zcash Login entry to the Use Cases menu zechub-wiki#694