refactor: convert repository to monorepo with npm workspaces

Restructured the repository into a monorepo to better organize application code
and maintenance scripts.

## Workspace Structure
- web-app: Next.js application (all app code moved from root)
- housekeeping: Database backup and maintenance scripts

## Key Changes
- Moved all application code to web-app/ using git mv
- Moved database scripts to housekeeping/ workspace
- Updated Dockerfile for monorepo build process
- Updated docker-compose files (volume paths: ./web-app/etc/hosts/)
- Updated .gitignore for workspace-level node_modules
- Updated documentation (README.md, CLAUDE.md, CHANGELOG.md)

## Migration Impact
- Root package.json now manages workspaces
- Build commands delegate to web-app workspace
- All file history preserved via git mv
- Docker build process updated for workspace structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/app/ui/AddMonthButton.tsx

@@ -0,0 +1,30 @@
+"use client";
+
+import { PlusCircleIcon, CalendarDaysIcon } from "@heroicons/react/24/outline";
+import React from "react";
+import { formatYearMonth } from "../lib/format";
+import { YearMonth } from "../lib/db-types";
+import Link from "next/link";
+import { useLocale, useTranslations } from 'next-intl';
+
+export interface AddMonthButtonProps {
+    yearMonth: YearMonth;
+}
+
+export const AddMonthButton:React.FC<AddMonthButtonProps> = ({ yearMonth }) => {
+
+    const t = useTranslations("home-page.add-month-button");
+    const locale = useLocale();
+
+    return(
+        <div className="card card-compact shadow-s mb-4">
+            <Link href={`/${locale}/home/year-month/${formatYearMonth(yearMonth)}/add`} className='grid self-center tooltip' data-tip={t("tooltip")}>
+                <span className='flex self-center'>
+                    <CalendarDaysIcon className="h-[1em] w-[1em] cursor-pointer text-4xl"  />
+                    <PlusCircleIcon className="h-[1em] w-[1em] cursor-pointer text-xl text-green-500 ml-[-.4em] mt-[-.4em]"  />
+                    <span className="ml-1 self-center text-xs text-left leading-[1.2em] w-[6em]">{t("tooltip")}</span>
+                </span>
+            </Link>
+        </div>
+    );
+}