feat: standardise api error response

[xrvnd]

Summary

• Add ERROR_CODES constants (1001–1005) and a shared ApiResponse interface so all routes return { data, error?: { code, message } } --> (need more review attention on these error labels please)
• Update ALL 4 backend routes (/puzzle/status, /puzzle/check, /puzzle/hint, /new-user) to use the standard shape with correct HTTP status codes
• Update frontend to read actual error messages from the response instead of our hard coded strings - "Database is inactive..."

Test plan

• Game loads normally when DB is active
• When DB is inactive, toast shows "Internal server error" (not the old string)
• Hint and guess flows still work correctly

Closes #36

[vercel]

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[coder-zs-cse]

Pls solve the merge conflicts