fix(ux): modal error dialog for server-refused actions + ESC closes only the topmost

- Duplicate-tag create/rename hit the uq_tags_user_name constraint and 500'd; now caught and
  returned as a clear 409 ('You already have a tag named …').
- New global error dialog (errorDialog store + ErrorDialog modal) for definitive server refusals;
  the api layer wiring + mount land with the rest of the round.
- Modal now keeps a stack so ESC dismisses only the top modal — an error over the tag editor
  closes the error and leaves the editor open.

backend/app/routes/tags.py

@@ -1,5 +1,6 @@
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import func, or_, select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app.auth import current_user
@@ -49,7 +50,11 @@ def create_tag(
         category=category if category in ("language", "topic", "other") else "other",
     )
     db.add(tag)
-    db.commit()
+    try:
+        db.commit()
+    except IntegrityError:
+        db.rollback()
+        raise HTTPException(status_code=409, detail=f"You already have a tag named “{name}”.")
     return {"id": tag.id, "name": tag.name, "color": tag.color, "category": tag.category}
 
 
@@ -76,7 +81,11 @@ def update_tag(
         tag.name = name
     if "color" in payload:
         tag.color = payload.get("color")
-    db.commit()
+    try:
+        db.commit()
+    except IntegrityError:
+        db.rollback()
+        raise HTTPException(status_code=409, detail=f"You already have a tag named “{tag.name}”.")
     return {"id": tag.id, "name": tag.name, "color": tag.color, "category": tag.category}