fix(import): explain oversized SQL import requests

[mlhiter]

Summary

SQL file and text imports now show a localized size-limit message when the server rejects a request with HTTP 413, while ordinary SQL errors retain their original messages.
Multipart uploads preserve status 413 for plain-text, HTML, and empty responses, with regression coverage for both import modes.

Validation

Passed frontend typecheck, production build, ESLint, all 209 tests, backend go build ./..., and the pre-commit secret scan; the frontend build reported non-blocking PostCSS and bundle warnings.

Sequence Diagram

sequenceDiagram
    participant User
    participant Modal as DatabaseImportModal
    participant Client as GraphQL Client
    participant Server
    User->>Modal: Import SQL file or text
    Modal->>Client: ImportSQL mutation
    Client->>Server: Send import request
    alt HTTP 413
        Server-->>Client: Reject oversized request
        Client-->>Modal: Network error with statusCode 413
        Modal-->>User: Localized size-limit message
    else Ordinary SQL error
        Server-->>Client: SQL error
        Client-->>Modal: Original error
        Modal-->>User: Original error message
    end

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