Fix unit/product kind enums serializing as integers

Two bugs found during exploratory testing:

1. UnitKind and ProductKind enums were serialized as 0/1 instead of
   "Global"/"Family" because no global JSON converter was registered.
   Added JsonStringEnumConverter to ConfigureHttpJsonOptions so all
   enum responses (kind, category) serialize as strings. This also
   fixes UnitCategory coming back as a number.

2. units.svelte.ts triggered a Svelte 5 state_unsafe_mutation error
   because the `all` getter called load() (which mutates $state) from
   inside a $derived expression in QuantityInput. Wrapped the load()
   call in untrack() so the side-effect runs outside the reactive
   tracking context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/backend/YesChef.Api/Program.cs

@@ -1,5 +1,6 @@
 using System.Security.Claims;
 using System.Text;
+using System.Text.Json.Serialization;
 using System.Threading.RateLimiting;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
 using Microsoft.AspNetCore.RateLimiting;
@@ -20,6 +21,9 @@ using YesChef.Api.Features.Units;
 
 var builder = WebApplication.CreateBuilder(args);
 
+builder.Services.ConfigureHttpJsonOptions(o =>
+    o.SerializerOptions.Converters.Add(new JsonStringEnumConverter()));
+
 builder.Services.AddDbContext<YesChefDb>(options =>
     options.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection")));
 

src/frontend/src/lib/units.svelte.ts

@@ -1,3 +1,4 @@
+import { untrack } from 'svelte';
 import { api } from '$lib/api';
 
 export type UnitKind = 'Global' | 'Family';
@@ -24,7 +25,10 @@ let error = $state<string | null>(null);
 export const units = {
 	get all() {
 		if (cache === null && !loading) {
-			void load();
+			// Use untrack so the load() side-effect (which mutates $state) is
+			// not flagged as an illegal mutation when this getter is read inside
+			// a $derived expression.
+			untrack(() => void load());
 		}
 		return cache ?? [];
 	},