fix: honor configured frontend API version (#502)

[mertsparlak]

• Honor VITE_API_VERSION when generating frontend runtime configuration instead of always rebuilding the API path from legacy variables.
• Derive VITE_API_START and VITE_API_VERSION_ONLY from the configured path to keep API requests, widgets, and execution streams consistent.
• Preserve the legacy variable fallback and default /api/v1 path when no explicit API version is provided.
• Normalize leading and trailing slashes and align the shared frontend configuration reader with the runtime configuration.
• Verify twelve configuration scenarios, frontend typecheck, and production build; lint remains blocked by the existing ESLint/AJV initialization error, also reproduced without this change.

Related: #502