refactor(app): DRY pass — split large files, extract shared utils

S1: delete page.new.js (1724L abandoned) + remove dead getAntigravityProjectId
S2: split large files by natural seams
  - usage.js → usage/{github,google,claude,codex,kiro,minimax,misc,shared}.js
  - media-providers page → components/{Embedding,Tts,Generic,Stt}ExampleCard.js
  - EndpointPageClient → endpointConstants.js + endpointPing.js + components/
  - tokenRefresh.js → tokenRefresh/{dedup,providers}.js
  - ProviderLimits/index.js: 16 pure fn + 9 constants → utils.js
  - oauth/providers.js: 7 pure helpers → providerHelpers.js
S3: shared utils
  - getModelKind(m, fallback) → shared/constants/models.js (replaces 20× m.kind||m.type)
  - getStatusVariant → shared/utils/connectionStatus.js (dedup ConnectionRow/ConnectionsCard)
  - sseChunk → open-sse/utils/sse.js (dedup grok-web/perplexity-web)
  - fetchWithTimeout → usage/shared.js (replace 4× AbortController pattern in google.js)
fix: enableObservability2 field name in requestDetailsRepo
Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/executors/perplexity-web.js

@@ -1,6 +1,7 @@
 import { BaseExecutor } from "./base.js";
 import { PROVIDERS } from "../config/providers.js";
 import { SSE_DONE, SSE_HEADERS_NO_BUFFER } from "../utils/sseConstants.js";
+import { sseChunk } from "../utils/sse.js";
 
 const PPLX_SSE_ENDPOINT = PROVIDERS["perplexity-web"].baseUrl;
 const PPLX_API_VERSION = "2.18";
@@ -290,10 +291,6 @@ async function* extractContent(eventStream, signal) {
   yield { delta: "", answer: fullAnswer, backendUuid: backendUuid ?? undefined, done: true };
 }
 
-function sseChunk(data) {
-  return `data: ${JSON.stringify(data)}\n\n`;
-}
-
 function buildStreamingResponse(eventStream, model, cid, created, history, currentMsg, signal) {
   const encoder = new TextEncoder();
   return new ReadableStream({