Add Zod schema validation for /api/features response

## Context

PR #9094 introduced `serverCapabilities.ts` which fetches server capabilities from `GET /api/features` and stores the JSON response directly. Currently no runtime schema validation is performed on the response.

## Problem

If the server returns malformed data (e.g. `supports_preview_metadata: "yes"` instead of `true`), the invalid value propagates to consumers. While `getServerCapability(key, defaultValue)` provides fallback defaults, type mismatches can still cause subtle bugs in boolean checks where truthy strings pass.

## Proposal

Add a Zod schema to validate the `/api/features` response:
- Define expected shape and types for known capability keys
- Use `safeParse` — on failure, log a warning and fall back to `EMPTY`
- Allow unknown keys to pass through (`.passthrough()`) for forward compatibility

## Related

- Part of #9079
- Raised in PR #9094 review ([comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/9094#discussion_r2867431083))

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