Checkpoint: Initial stable CheckFlow base before i18n and demo mode

public/icons/icon-192.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100"><rect width="100" height="100" rx="20" fill="#4B7BF5"/><text x="50" y="67" font-family="Arial" font-size="55" font-weight="bold" fill="white" text-anchor="middle">&#x2713;</text></svg>

public/icons/icon-512.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100"><rect width="100" height="100" rx="20" fill="#4B7BF5"/><text x="50" y="67" font-family="Arial" font-size="55" font-weight="bold" fill="white" text-anchor="middle">&#x2713;</text></svg>

public/icons/icon.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100"><rect width="100" height="100" rx="20" fill="#4B7BF5"/><text x="50" y="67" font-family="Arial" font-size="55" font-weight="bold" fill="white" text-anchor="middle">&#x2713;</text></svg>

public/manifest.json

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+{"name":"CheckFlow","short_name":"CheckFlow","description":"Clean, fast self-hosted todo app","start_url":"/","display":"standalone","background_color":"#FFFFFF","theme_color":"#4B7BF5","orientation":"any","icons":[{"src":"/icons/icon.svg","sizes":"any","type":"image/svg+xml","purpose":"any maskable"}],"categories":["productivity","utilities"],"lang":"ko"}

public/sw.js

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+const CACHE_NAME = "checkflow-v1";
+const STATIC_ASSETS = ["/", "/login", "/register", "/manifest.json", "/icons/icon.svg"];
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => cache.addAll(STATIC_ASSETS))
+  );
+  self.skipWaiting();
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(keys.filter((k) => k !== CACHE_NAME).map((k) => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener("fetch", (event) => {
+  const { request } = event;
+  const url = new URL(request.url);
+
+  // Network-first for API
+  if (url.pathname.startsWith("/api")) {
+    event.respondWith(fetch(request).catch(() => new Response(JSON.stringify({ error: "Offline" }), { headers: { "Content-Type": "application/json" } })));
+    return;
+  }
+
+  // Cache-first for static
+  event.respondWith(
+    caches.match(request).then((cached) => cached || fetch(request))
+  );
+});