refactor(frontend): port api-docs/endpoints to TypeScript

endpoints.js was the only remaining JS file under src/. It's a pure data
file describing every panel API surface for the in-panel Swagger docs;
scripts/build-openapi.mjs reads it at build time to emit
public/openapi.json.

Convert it to endpoints.ts with explicit interfaces:
  HttpMethod, ParamLocation, ParamType,
  EndpointParam, Endpoint, SubscriptionHeader, Section

Type-checking surfaced shapes the .js had silently accepted:
  - 'in' values beyond plain 'body' — 'body (form)', 'body (json)',
    'body (multipart)' for non-JSON request bodies
  - 'type' arrays — 'integer[]', 'object[]'
  - Subscription section's subHeader documenting response headers
All four are now part of the union types so the existing data type-checks.

Dead exports removed:
  - safeInlineHtml — unused since the docs page switched to Swagger UI
  - methodColors — unused

Build pipeline:
  - scripts/build-openapi.mjs imports endpoints.ts directly
  - gen:api runs via Node 22's native --experimental-strip-types; no
    tsx/ts-node dependency added
  - --disable-warning=ExperimentalWarning silences just the strip-types
    notice while keeping deprecation warnings intact

frontend/scripts/build-openapi.mjs

@@ -3,7 +3,7 @@ import { writeFileSync } from 'node:fs';
 import { join, dirname } from 'node:path';
 import { fileURLToPath, pathToFileURL } from 'node:url';
 
-import { sections } from '../src/pages/api-docs/endpoints.js';
+import { sections } from '../src/pages/api-docs/endpoints.ts';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const outPath = join(__dirname, '..', 'public', 'openapi.json');