feat: brain masonry card grid — Karakeep-style layout with Atelier aesthetics

- 3-column CSS masonry grid (2 on tablet, 1 on mobile)
- Link cards show screenshot thumbnails
- Note cards show content body inline
- Tags as pills, folder/date in meta footer
- Screenshot serving endpoint added to brain API
- Auto-polling for pending items (3s interval)
- Detail sheet shows raw_content for notes
- Warm frosted glass card styling matching Atelier design

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/brain/app/models/schema.py

@@ -68,6 +68,8 @@ class ItemOut(BaseModel):
     type: str
     title: Optional[str] = None
     url: Optional[str] = None
+    raw_content: Optional[str] = None
+    extracted_text: Optional[str] = None
     folder: Optional[str] = None
     tags: Optional[list[str]] = None
     summary: Optional[str] = None