feat: import transactions from a CSV bank statement

Fourth of the four ideas picked from the Maybe/Sure feature audit, and the
last one queued up. A 3-step wizard (account -> column mapping -> preview)
at /transactions/import, parsed entirely client-side (new lib/csv.ts —
delimiter auto-detect, 5 date formats, thousands/decimal-separator-aware
amounts) and submitted through a new all-or-nothing POST /transactions/bulk
endpoint, mirroring backup restore's validate-everything-before-touching-
anything approach.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/TransactionsPage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from "react";
-import { useSearchParams } from "react-router-dom";
-import { Plus, Search, X } from "lucide-react";
+import { Link, useSearchParams } from "react-router-dom";
+import { FileUp, Plus, Search, X } from "lucide-react";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/Card";
 import { Button } from "@/components/ui/Button";
 import { Input, Select } from "@/components/ui/Input";
@@ -137,10 +137,18 @@ export function TransactionsPage() {
             />
           </div>
         </div>
-        <Button onClick={openCreateModal} className="w-full sm:w-auto">
-          <Plus size={16} />
-          {t("transactions.addButton")}
-        </Button>
+        <div className="flex gap-2">
+          <Link to="/transactions/import" className="flex-1 sm:flex-none">
+            <Button variant="secondary" className="w-full sm:w-auto">
+              <FileUp size={16} />
+              {t("transactions.importButton")}
+            </Button>
+          </Link>
+          <Button onClick={openCreateModal} className="flex-1 sm:w-auto">
+            <Plus size={16} />
+            {t("transactions.addButton")}
+          </Button>
+        </div>
       </div>
 
       <div className="relative">