fix(frontend): check Freighter availability on startup

[mathewsap45]

Summary

Closes #682

Adds a Freighter extension availability check in frontend/src/main.ts that runs after the app mounts.

Problem

Components depending on Freighter would fail silently if the extension wasn't installed, with no user-facing feedback.

Fix

• isFreighterAvailable() checks for window.freighter
• showFreighterBanner() injects a fixed top banner with a link to https://www.freighter.app/ when Freighter is not detected
• Check runs after app mount so the DOM is ready

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