feat: add recurring invoices, public links, time tracker, payments, reminders

- Recurring invoices: schedule-based auto-generation with CRUD UI at /dashboard/invoices/recurring and POST /api/cron/generate-recurring cron endpoint
- Public invoice link: generate/revoke shareable /i/[token] page for unauthenticated clients with PDF download
- Live time tracker: localStorage-persisted timer widget in invoice editor that appends a line item on stop (rounds to nearest 0.25h)
- Partial payment tracking: record payments per invoice, auto-mark paid when fully covered, balance due display
- Send reminder: email reminder via Resend with custom message dialog and last-sent indicator

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

src/app/api/cron/generate-recurring/route.ts

@@ -0,0 +1,16 @@
+import { type NextRequest, NextResponse } from "next/server";
+import { env } from "~/env";
+import { db } from "~/server/db";
+import { generateDueRecurringInvoices } from "~/server/api/routers/recurring-invoices";
+
+export async function POST(req: NextRequest) {
+  const authHeader = req.headers.get("authorization");
+  const secret = env.CRON_SECRET;
+
+  if (secret && authHeader !== `Bearer ${secret}`) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const generated = await generateDueRecurringInvoices(db);
+  return NextResponse.json({ generated });
+}