feat(vault): open any CID from a link or paste, with trustless-gateway fallback #54

Summary

Add a page that resolves a CID that is not in the local blockstore, so shared results open without the sender's browser.

Estimated size: M

Motivation

Today a CID shared with another user resolves only if they obtain the block by other means.

Current state in this repo

• Helia runs with createHelia({ blockstore, datastore }) and no custom libp2p or block-broker configuration (features/ipfs/lib/helia-init.ts).

Proposal

• Add /vault/cid/[cid] with a paste box.
• Configure the trustless-gateway block broker with a configurable gateway list (env).
• Show a verified-against-CID indicator and cache fetched blocks locally.
• Validate the CID and the decoded record schema.

Acceptance criteria

• A CID published and pinned elsewhere opens on a fresh browser profile.
• Invalid CIDs and unreachable gateways give distinct messages.
• A fetched block is verified against its CID before display.

Out of scope

• Browser-to-browser transports (see the Kubo browser-connectivity issue).

Technical notes and risks

• Gateway choice affects privacy (the gateway sees the CID); document it.

Dependencies

• None; can be picked up independently.

Testing plan

• Mocked gateway tests; one manual cross-profile check.

Part of #43. Ground rules for the content layer: no secrets in the repo, content-layer failures must never block or discard quantum execution, and tests and docs ship with every PR.