Add smart ACK provider for WeChat MP replies

Replace fixed ackText with a rule-based AckProvider that picks
response templates by message type and intent (translate, summary,
rewrite, poster, ppt, mindmap, code, search, schedule). Pure sync,
zero I/O, auto-falls back to config.ackText on any error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/dev/mindspacePreviewData.ts

@@ -25,7 +25,7 @@ export const PREVIEW_SPACE: MindSpace = {
     usedBytes: 1.2 * 1024 * 1024,
     reservedBytes: 0,
     availableBytes: 3.8 * 1024 * 1024,
-    maxFileBytes: 2 * 1024 * 1024,
+    maxFileBytes: 5 * 1024 * 1024,
     publicPageLimit: 3,
     publicPageUsed: 1,
     aiDailyLimit: 20,