refactor(app): RISKY pass R1-R3 — config-driven modal, cursor frame dedup, chunk helper

R1: merge AddOpenAICompatibleModal + AddAnthropicCompatibleModal → AddCompatibleModal (variant config-driven, ~180 dup removed, preserves per-variant useEffect behavior)
R3: extract readCursorFrame() helper — dedup protobuf frame header/decompress loop (JSON+SSE transforms, byte-identical)
R2: add chatChunkSse() helper, wire 7 cursor SSE scaffolds (byte-identical, cursor golden pass)
Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/utils/sse.js

@@ -1,3 +1,14 @@
 export function sseChunk(data) {
   return `data: ${JSON.stringify(data)}\n\n`;
 }
+
+// Build OpenAI chat.completion.chunk SSE frame. Key order: id, object, created, model, choices.
+export function chatChunkSse({ id, created, model, delta, finishReason = null }) {
+  return sseChunk({
+    id,
+    object: "chat.completion.chunk",
+    created,
+    model,
+    choices: [{ index: 0, delta, finish_reason: finishReason }],
+  });
+}