feat: complete channel menu install flow + XiaoHongShu channel

Major install flow polish:
- Channels organized by setup difficulty:
  🟢 Zero-config (Web, GitHub, RSS) — works immediately
  🔵 Cookie-based (Twitter, YouTube, Bilibili, XiaoHongShu) — free, ~2min
  🟡 Free API key (Exa Search) — one key, 30 seconds
  🟠 Proxy-based (Reddit, Bilibili on server) — $1/month
- Every channel explains: what it does, what's needed, what you miss without it
- Server vs local affects which channels need proxy

New: XiaoHongShu channel (cookie-based, falls back to Jina Reader)
New configure commands: twitter-cookies, xhs-cookie, youtube-cookies
Each command auto-tests after saving.

agent_eyes/channels/__init__.py

@@ -19,6 +19,7 @@ from .reddit import RedditChannel
 from .rss import RSSChannel
 from .bilibili import BilibiliChannel
 from .exa_search import ExaSearchChannel
+from .xiaohongshu import XiaoHongShuChannel
 
 
 # Channel registry — order matters (first match wins, web is last as fallback)
@@ -28,6 +29,7 @@ ALL_CHANNELS: List[Channel] = [
     YouTubeChannel(),
     RedditChannel(),
     BilibiliChannel(),
+    XiaoHongShuChannel(),
     RSSChannel(),
     ExaSearchChannel(),
     WebChannel(),        # Fallback — handles any URL