feat(w3up-client): switch browser agent keys to ed25519

[NiKrause]

Implements new ed25519 WebCrypto model behaviour (#658)

Key runtime changes:

• Browser client now generates Ed25519 principals by default instead of RSA.
  • packages/w3up-client/src/index.js now imports @ucanto/principal/ed25519.
• Node client keeps explicit extractable Ed25519 generation for Conf-backed persistence.
  • packages/w3up-client/src/index.node.js now calls generate({ extractable: true }).
• Space key generation for mnemonic-capable owned spaces is explicit extractable Ed25519.
  • packages/access-client/src/space.js generate() now uses ED25519.generate({ extractable: true }).
• Mnemonic export now fails fast with a clear error when a signer is non-extractable.
  • Added explicit TypeError in packages/access-client/src/space.js.
• Conf driver now rejects non-extractable Ed25519 CryptoKey archives with a clear message.
  • Prevents silently persisting unsupported JSON payloads in Node conf storage.

Test updates:

• Updated browser key expectations from RSA to Ed25519:
  • packages/w3up-client/test/index.browser.test.js
• Updated Node BYO principal tests to use extractable keys for Conf-store compatibility:
  • packages/w3up-client/test/index.node.test.js
• Added explicit non-extractable mnemonic error coverage:
  • packages/w3up-client/test/space.test.js
  • packages/access-client/test/agent.test.js
• Expanded store coverage for ed25519 persistence semantics:
  • packages/access-client/test/stores/store-indexeddb.browser.test.js
  • packages/access-client/test/stores/store-conf.node.test.js

Tooling/test infra updates:

• Upgraded workspace playwright-test catalog to ^14.0.0 (resolved to 14.1.12).
• Regenerated pnpm lockfile.
• This resolves local browser startup crashes seen with older playwright-core/chromium revisions and keeps browser tests aligned with new key expectations.

Before merging this PR - ucanto PR must be merged first storacha/ucanto#408