fix: report a duplicate name instead of failing the page

Adding a category or project whose name exists, or renaming an item onto an
existing name, hit the unique constraint and rendered SvelteKit's 500 page.
Translate SQLITE_CONSTRAINT_UNIQUE into a form error on the three forms.

6 files changed, 86 insertions, 5 deletions

diff --git a/README.md b/README.md
index 936453a..f3f1321 100644
--- a/README.md
+++ b/README.md
@@ -81,8 +81,6 @@ mise run deploy
 
 ## TODO
 
-- Duplicate-name forms return a 500 (catch the unique constraint on
-  groceries, categories and projects)
 - Purchase history (`checked_at` → "acheté il y a 3 jours", catalog ordered by
   last bought)
 - Automated database backup (cron + retention + a restore drill)
@@ -94,3 +92,4 @@ mise run deploy
 - Auto-categorize new items (LLM-assisted)
 - https://automerge.org/docs/tutorial/
 - Shared > Modal actions
+- Result type over throws
diff --git a/src/lib/server/db/errors.test.ts b/src/lib/server/db/errors.test.ts
new file mode 100644
index 0000000..d19aec2
--- /dev/null
+++ b/src/lib/server/db/errors.test.ts
@@ -0,0 +1,45 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { groceries, categories } from '$lib/server/db/schema';
+import { eq } from 'drizzle-orm';
+import { createTestDb, type TestDb } from '$lib/server/db/test-helper';
+import { isUniqueViolation } from './errors';
+
+describe('isUniqueViolation', () => {
+	let db: TestDb['db'];
+
+	beforeEach(async () => {
+		db = (await createTestDb()).db;
+		await db.insert(categories).values({ name: 'Épices' });
+		await db
+			.insert(groceries)
+			.values({ name: 'olives', createdAt: '2026-01-01T00:00:00.000Z' });
+	});
+
+	it('detects a duplicate insert on a unique column', async () => {
+		const error = await db
+			.insert(categories)
+			.values({ name: 'Épices' })
+			.catch((e: unknown) => e);
+
+		expect(isUniqueViolation(error)).toBe(true);
+	});
+
+	it('detects a duplicate name on update', async () => {
+		await db
+			.insert(groceries)
+			.values({ name: 'pain', createdAt: '2026-01-02T00:00:00.000Z' });
+
+		const error = await db
+			.update(groceries)
+			.set({ name: 'olives' })
+			.where(eq(groceries.name, 'pain'))
+			.catch((e: unknown) => e);
+
+		expect(isUniqueViolation(error)).toBe(true);
+	});
+
+	it('leaves other failures alone', () => {
+		expect(isUniqueViolation(new Error('network is unreachable'))).toBe(false);
+		expect(isUniqueViolation(undefined)).toBe(false);
+	});
+});
diff --git a/src/lib/server/db/errors.ts b/src/lib/server/db/errors.ts
new file mode 100644
index 0000000..4b101f8
--- /dev/null
+++ b/src/lib/server/db/errors.ts
@@ -0,0 +1,13 @@
+/**
+ * True when the failure is SQLite's UNIQUE constraint. Drizzle wraps driver
+ * errors, so the libsql error sits at some depth of the `cause` chain.
+ */
+export function isUniqueViolation(error: unknown): boolean {
+	let current: unknown = error;
+	while (current instanceof Error) {
+		if ('code' in current && current.code === 'SQLITE_CONSTRAINT_UNIQUE')
+			return true;
+		current = current.cause;
+	}
+	return false;
+}
diff --git a/src/lib/server/services/grocery-actions.ts b/src/lib/server/services/grocery-actions.ts
index 1cfb05d..cedb46e 100644
--- a/src/lib/server/services/grocery-actions.ts
+++ b/src/lib/server/services/grocery-actions.ts
@@ -2,6 +2,7 @@ import { fail, redirect, type ActionFailure } from '@sveltejs/kit';
 import { addOrUncheck, loadUnchecked, toggle } from './groceries';
 import { add as addCategory } from './categories';
 import { numericId, parseFormString, parseOptionalNumber } from './validation';
+import { isUniqueViolation } from '$lib/server/db/errors';
 
 export async function toggleGroceryAction(request: Request) {
 	const data = await request.formData();
@@ -50,7 +51,14 @@ export async function addCategoryAction(request: Request) {
 		return name;
 	}
 
-	await addCategory(name);
+	try {
+		await addCategory(name);
+	} catch (error) {
+		if (isUniqueViolation(error)) {
+			return fail(400, { error: 'Cette catégorie existe déjà' });
+		}
+		throw error;
+	}
 	return { success: true };
 }
 
diff --git a/src/routes/+page.server.ts b/src/routes/+page.server.ts
index fe9b4e3..008f7f3 100644
--- a/src/routes/+page.server.ts
+++ b/src/routes/+page.server.ts
@@ -2,6 +2,7 @@ import { fail } from '@sveltejs/kit';
 import { loadOverview } from '$lib/server/services/routines';
 import { add as addProject } from '$lib/server/services/projects';
 import { parseFormString } from '$lib/server/services/validation';
+import { isUniqueViolation } from '$lib/server/db/errors';
 import type { Actions, PageServerLoad } from './$types';
 
 export const load: PageServerLoad = async () => {
@@ -14,7 +15,14 @@ export const actions: Actions = {
 		if (!name) {
 			return fail(400, { error: 'Le nom est requis' });
 		}
-		await addProject(name);
+		try {
+			await addProject(name);
+		} catch (error) {
+			if (isUniqueViolation(error)) {
+				return fail(400, { error: 'Ce projet existe déjà' });
+			}
+			throw error;
+		}
 		return { success: true };
 	},
 };
diff --git a/src/routes/grocery/[id]/+page.server.ts b/src/routes/grocery/[id]/+page.server.ts
index a4ad377..232568c 100644
--- a/src/routes/grocery/[id]/+page.server.ts
+++ b/src/routes/grocery/[id]/+page.server.ts
@@ -16,6 +16,7 @@ import {
 	parseFormString,
 	parseOptionalNumber,
 } from '$lib/server/services/validation';
+import { isUniqueViolation } from '$lib/server/db/errors';
 import type { Category, Grocery, Reference } from '$lib/server/services/types';
 import type { Actions, PageServerLoad } from './$types';
 
@@ -86,7 +87,14 @@ export const actions: Actions = {
 			return fail(400, { error: 'Le nom est requis' });
 		}
 
-		await updateGrocery(groceryId, name, categoryId);
+		try {
+			await updateGrocery(groceryId, name, categoryId);
+		} catch (error) {
+			if (isUniqueViolation(error)) {
+				return fail(400, { error: 'Un article porte déjà ce nom' });
+			}
+			throw error;
+		}
 		return { success: true, type: 'update' as const };
 	},