Handle Freighter wallet not-installed error with a user-friendly message

[Kingsman-99]

Description

FreighterAdapter calls the Freighter API directly and propagates raw TypeError: Cannot read properties of undefined errors when the extension is not installed. Users see a cryptic crash instead of an actionable message.

Acceptance Criteria

• The adapter checks for window.freighter (or the equivalent global) before any API call
• When Freighter is absent, a FreighterNotInstalledError is thrown with the Freighter install URL in the message
• Existing connection logic is unchanged when the extension is present
• Unit tests pass

Context

• Target file: src/wallets/adapters/FreighterAdapter.ts
• The install URL is https://www.freighter.app