fix(desktop): restrict external links

[opencode-agent]

Summary

• validate renderer-provided external URLs in the main process
• allow only HTTP and HTTPS links before calling shell.openExternal
• reject file, executable/custom-protocol, and malformed URLs

Checks

• bun test src/main/external-link.test.ts
• bun typecheck

Requested by: @Brendonovich (Brendan Allan via Slack)

[github-actions]

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