chore(web): fix prefer-const and remove unused import

Three pre-existing lint issues surfaced during #70's lint check:
- lists/[id]/page.tsx: subtasksMap was `let`, never reassigned -> const
- api/lists/[id]/tasks/route.ts: query was `let`, never reassigned -> const
- Header.tsx: drop unused `Link` import from next/link

Leaves web/ lint at a single remaining error (ThemeToggle
set-state-in-effect), tracked in #90 (needs a design decision:
useSyncExternalStore vs documented eslint-disable).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/(app)/lists/[id]/page.tsx

@@ -74,7 +74,7 @@ export default async function ListPage({
 
   // Fetch subtasks for all parent tasks
   const parentIds = tasks.map((t) => t.id);
-  let subtasksMap: Record<string, Task[]> = {};
+  const subtasksMap: Record<string, Task[]> = {};
 
   if (parentIds.length > 0) {
     const allSubtasks = await db

web/src/app/api/lists/[id]/tasks/route.ts

@@ -49,7 +49,7 @@ export async function GET(
   }
 
   // Build query
-  let query = db
+  const query = db
     .select()
     .from(slTasks)
     .where(and(...conditions));

web/src/components/Header.tsx

@@ -2,7 +2,6 @@
 
 import { ThemeToggle } from "./ThemeToggle";
 import { User, LogOut } from "lucide-react";
-import Link from "next/link";
 import { useState } from "react";
 import { useTranslation } from "react-i18next";