feat: group posts by batch in archive + fullscreen confirm modal

Backend:
- Add batch_id column to Post model (UUID, groups posts from same generation)
- Set batch_id in /generate endpoint for all posts in same request

Frontend:
- ContentArchive: group posts by batch_id into single cards with platform tabs
- Character name at top, platform tabs below, status badge, text preview
- Click platform tab to switch between variants of same content
- ConfirmModal: render via React portal to document.body for true fullscreen overlay
- Add box-shadow and higher z-index for better visual separation

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

backend/app/routers/content.py

@@ -3,6 +3,7 @@
 Handles post generation via LLM, image generation, and CRUD operations on posts.
 """
 
+import uuid
 from datetime import date, datetime
 
 from fastapi import APIRouter, Depends, HTTPException, Query
@@ -132,7 +133,8 @@ def generate_content(
             for link in links
         ]
 
-    # Generate one post per platform
+    # Generate one post per platform, all sharing the same batch_id
+    batch_id = str(uuid.uuid4())
     posts_created: list[Post] = []
     for platform in platforms:
         text = generate_post_text(
@@ -152,6 +154,7 @@ def generate_content(
             )
 
         post = Post(
+            batch_id=batch_id,
             character_id=character.id,
             user_id=current_user.id,
             content_type=request.content_type,