fix(explorer): provide Privy development defaults for previews

Parent tracker: #436
Owning PR: #429
Depends on: #442 / PR #426

## Goal

Let local and preview builds use documented public Privy development identifiers when explicit environment values are absent.

## Scope

- Provide public development defaults for the Privy application and client identifiers.
- Preserve explicit environment overrides.
- Document the development fallback behavior.
- Keep production provisioning tracked separately by #382.

## Non-goals

- Treating public application identifiers as secrets.
- Supplying production Privy configuration.
- Restacking, merging, or otherwise changing PR state.

## Acceptance criteria

- Local and preview builds can initialize Privy without private configuration.
- Explicit environment values always override the defaults.
- The fallback is clearly limited to development use.
- Focused configuration tests cover default and override behavior.

## Performance

No material performance impact is expected.

## Delivery

PR #429 remains based on PR #426 while the stack is open. After its predecessor merges, retarget it to staging and revalidate the exact head.

The issue names no files, entry points, or test paths; start by locating the Privy configuration and its existing environment-value handling. Review PR #429 and its dependency on PR #426 before making changes. Done means development defaults work for local and preview builds, explicit values override them, the fallback is documented, and focused configuration tests cover both cases.

Written by the indexing model from the issue text.