Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
packages/ui/src/features/canvas/components/WebsiteLayout.tsx:45
**Parallel registry that can drift from `BUILT_INS`**

`DATA_TEMPLATES` is a second, hand-maintained list that must stay in sync with `canvasTemplates.ts`'s `BUILT_INS`. The PR's own `AI_GATEWAY.md` notes this: "That's a parallel list to the core template registry — worth collapsing into a `dataTemplate` flag on the template record so the two can't drift." A future data-driven template will require the same two-file update in different packages, and there is nothing to catch it if one side is missed. Adding a `dataTemplate?: boolean` field to `BuiltInTemplate` and deriving this list from the registry would remove the duplication.

### Issue 2 of 3
packages/core/src/canvas/canvasTemplates.test.ts:44-50
The "connect snippets" test bundles four independent `toContain` assertions in one non-parameterised `it`. The formula test just above it already uses `it.each` for exactly this pattern, and the team preference is parameterised tests throughout. Applying the same shape here keeps the test suite consistent and gives a descriptive failure message per assertion.

```suggestion
  it.each([
    ["OpenAI base URL", "baseURL: '<gateway base URL>/v1'"],
    ["Anthropic SDK import", "@anthropic-ai/sdk"],
    ["provider state path", '"$state": "/provider"'],
    ["language state path", '"$state": "/language"'],
  ])("bakes the %s into the connect section", (_name, snippet) => {
    expect(aiGateway?.systemPrompt).toContain(snippet);
  });
```

### Issue 3 of 3
packages/core/src/canvas/canvasTemplates.ts:150-155
**Fragile string surgery on `GATEWAY_WHERE`**

The empty-state probe query is constructed by calling `.replace()` twice on `GATEWAY_WHERE`. If the constant is ever restructured (e.g. the placeholder renamed, or the `AND timestamp < {date_to}` clause moved), the replacement silently produces wrong SQL with no compile-time or test-time signal. No existing test asserts the assembled empty-state query text. A named constant — `const GATEWAY_EMPTY_STATE_WHERE = \`${GATEWAY_BASE_FILTER} AND timestamp >= now() - INTERVAL 30 DAY\`` — next to `GATEWAY_WHERE` would be self-documenting and immune to structural drift.