Expose the discovered wallet list and a useParties() hook

[fernandomg]

User story / Problem statement

Nothing exposes the wallets discovery found, or which one is connected. #3's acceptance criterion
"useConnect/CantonConnectProvider reflect the discovered wallet list and current selection" was
left unmet in #47: the list reaches CantonConnectConfig.walletPicker and nowhere else.

Two things need it:

• A themed in-page picker (companion issue) has to render the entries, so it cannot be built
  without them.
• Multi-account UI. useParty() returns the primary party only. A dApp showing a party switcher —
  the vesting app already has one — has to call sdk.listAccounts() itself and map the results. feat: adopt the dapp-sdk facade in canton-connect #47
  deliberately un-exported selectPrimaryAccount/toParty rather than make our internal mapping
  public API, on the grounds that the right answer is a hook we own.

Proposed solution

• useParties() returning every account mapped to Party, so a switcher needs no access to our
  mapping helpers and the toParty precedence rule stays ours to change.
• Expose the discovered entries and the current selection — most likely on the context value, since
  the picker is chosen before a session exists. Shape to be decided with the picker issue, so the two
  land coherently rather than inventing two overlapping surfaces.

Acceptance criteria

• useParties() exists, returns all accounts as Party, and updates on accountsChanged
• The discovered wallet entries and the current selection are readable from the public surface
• The themed picker can be built against it without reaching into the SDK directly
• selectPrimaryAccount and toParty stay internal — a consumer never needs them for this
• JSDoc on the new exports, per CLAUDE.md