Graceful offline fallback for crash report submission

[ricardo-rp]

Summary

When submitting crash or feedback reports, the itch desktop client attempts to upload the report to https://dale.itch.zone. If the report server is offline, returns an HTTP error, or cannot be reached from the renderer (throwing TypeError: Failed to fetch, as documented in #3067), the dialog currently shows an unformatted error stack without any way for users to retrieve their report or forward it to developers.

Changes

• Graceful Offline Fallback: If dale.itch.zone is offline or unreachable, the dialog informs the user and presents buttons to:
  • Copy report to clipboard: Copies the formatted message, system details, and application log with immediate visual feedback ("Copied to clipboard!").
  • File issue on GitHub: Opens https://github.com/itchio/itch/issues/new directly in the user's external browser.
• Preview in Dialog: Displays the formatted report in the preview block so users can inspect and copy their report even if clipboard access is restricted.
• Clean Error Handling: If the server returns a specific HTTP error or message, surfaces a human-readable summary instead of a raw stack trace.
• Preserved Existing Flow: If dale.itch.zone responds successfully (HTTP 200 with res.success), the upload flow succeeds and directs the user to their secret report URL as before.