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>