Improve update error handling with TTY detection

[gricha]

Summary

• Add detailed error reporting for perry update failures (504, 403, timeouts, etc.)
• Only show error messages in interactive TTY mode - silently exit in CI/scripts
• Increase fetch timeout from 3s to 5s to reduce false timeouts

Changes

src/update-checker.ts

• Added FetchVersionResult interface with version, error, and status fields
• Added fetchLatestVersionWithDetails() for detailed error info
• Kept fetchLatestVersion() as backward-compatible wrapper

src/index.ts

• Use process.stdout.isTTY to detect interactive mode
• Show specific messages for 504 (Gateway Timeout) and 403 (rate limit)
• In non-TTY mode, silently exit with code 1

Behavior

Scenario	TTY (interactive)	Non-TTY (scripts/CI)
504 error	Shows timeout message	Silent exit(1)
403 rate limit	Shows rate limit message	Silent exit(1)
Other errors	Shows error detail	Silent exit(1)
Success	Shows version info	Shows version info