Add category management UI and service layer

Users can now create, edit, and delete custom categories from Settings → Categories. System categories (45 built-in) are shown read-only. Backend adds update_category() and delete_category() service functions; frontend has a new categories API module and a full CRUD section in SettingsPage with filter tabs, colour picker, and delete confirmation.

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

backend/app/api/v1/categories.py

@@ -1,14 +1,28 @@
 import uuid
 
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel, Field
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.dependencies import get_current_user, get_db
-from app.services.category_service import create_category, list_categories
+from app.services.category_service import create_category, list_categories, update_category, delete_category
 
 router = APIRouter()
 
 
+class CategoryCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=100)
+    type: str = Field(..., pattern="^(income|expense|transfer)$")
+    color: str | None = Field(default=None, max_length=7)
+    icon: str | None = Field(default=None, max_length=50)
+
+
+class CategoryUpdate(BaseModel):
+    name: str | None = Field(default=None, min_length=1, max_length=100)
+    color: str | None = None
+    icon: str | None = None
+
+
 @router.get("")
 async def get_categories(
     db: AsyncSession = Depends(get_db),
@@ -19,18 +33,43 @@ async def get_categories(
 
 @router.post("", status_code=201)
 async def create(
-    body: dict,
+    body: CategoryCreate,
     db: AsyncSession = Depends(get_db),
     user=Depends(get_current_user),
 ):
     result = await create_category(
         db,
         user_id=user.id,
-        name=body["name"],
-        type_=body["type"],
-        icon=body.get("icon"),
-        color=body.get("color"),
-        parent_id=uuid.UUID(body["parent_id"]) if body.get("parent_id") else None,
+        name=body.name,
+        type_=body.type,
+        icon=body.icon,
+        color=body.color,
     )
     await db.commit()
     return result
+
+
+@router.put("/{category_id}", status_code=200)
+async def update(
+    category_id: uuid.UUID,
+    body: CategoryUpdate,
+    db: AsyncSession = Depends(get_db),
+    user=Depends(get_current_user),
+):
+    result = await update_category(db, user.id, category_id, body.name, body.color, body.icon)
+    if result is None:
+        raise HTTPException(status_code=404, detail="Category not found or is a system category")
+    await db.commit()
+    return result
+
+
+@router.delete("/{category_id}", status_code=204)
+async def delete(
+    category_id: uuid.UUID,
+    db: AsyncSession = Depends(get_db),
+    user=Depends(get_current_user),
+):
+    ok = await delete_category(db, user.id, category_id)
+    if not ok:
+        raise HTTPException(status_code=404, detail="Category not found or is a system category")
+    await db.commit()