Show vault metrics and the latest attested NAV

[luchobonatti]

User story / Problem statement

Investors judge the vault by its public figures, and by how recently it was valued.

Expected outcome

The vault's figures and the latest attested NAV are read from the chain.

Acceptance criteria

• Net assets, total supply and the remaining public figures render
• The latest attested NAV is labelled as attested and carries its timestamp
• No attestation sequence number is shown
• No unstruck price is presented as a quote
• Figures read as unavailable rather than zero when they cannot be fetched

Technical notes

Blocked by #64 and #65. Blocked by #84. Parent: #30.