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>

src/shared/constants/models.js

@@ -36,3 +36,5 @@ export function isValidModel(aliasOrId, modelId) {
 export const AI_MODELS = Object.entries(MODELS).flatMap(([alias, models]) =>
   models.map(m => ({ provider: alias, model: m.id, name: m.name }))
 );
+
+export const getModelKind = (m, fallback = null) => m?.kind || m?.type || fallback;