Add service worker implementation and tests for PWA caching strategy

- Implemented a service worker (sw.js) that handles installation, activation, and fetch events.
- Added caching logic for public assets and a fallback for offline navigation.
- Created unit tests (sw.test.ts) to validate service worker behavior, including asset caching, navigation handling, and activation logic.
- Ensured that private API routes and third-party requests are excluded from caching.

scripts/pwa-icons.ts

@@ -0,0 +1,17 @@
+import { createCanvas, loadImage } from "@napi-rs/canvas";
+import { mkdir } from "node:fs/promises";
+
+// Package the approved master at exact platform sizes; do not regenerate the artwork.
+const master = await loadImage("docs/design/mina-habits-icon.png");
+await mkdir("src/pwa/public", { recursive: true });
+for (const [name, size, scale] of [
+  ["icon-192", 192, 1], ["icon-512", 512, 1], ["icon-maskable-512", 512, 0.7],
+] as const) {
+  const canvas = createCanvas(size, size);
+  const ctx = canvas.getContext("2d");
+  ctx.fillStyle = "#000000";
+  ctx.fillRect(0, 0, size, size);
+  const inset = size * (1 - scale) / 2;
+  ctx.drawImage(master, inset, inset, size * scale, size * scale);
+  await Bun.write(`src/pwa/public/${name}.png`, canvas.toBuffer("image/png"));
+}