ihasmail 2.0: rebuild as Stalwart-first JMAP webmail

Replace the FastAPI/HTMX prototype with a Node/Hono session proxy and a
React 19/Vite SPA. Mail (conversation view, search operators, labels,
sanitised HTML, privacy image proxy, invites, undo send, templates),
calendar (month/week/day/agenda, invites, free/busy, categories,
context menus), contacts (JSContact, groups, vCard), files, Sieve filter
builder (incl. filter-from-message with retroactive apply), vacation,
identities with default + Reply-To, PWA/mobile layout, push via SSE,
in-memory mock Stalwart for dev, Docker + CI.

web/public/sw.js

@@ -0,0 +1,41 @@
+/* ihasmail service worker: app-shell caching for installability & fast loads.
+   API requests are never cached. */
+const VERSION = "ihasmail-v2";
+const SHELL = ["/", "/manifest.webmanifest", "/img/logo.png", "/img/icon-192.png", "/favicon.ico"];
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(caches.open(VERSION).then((c) => c.addAll(SHELL)).then(() => self.skipWaiting()));
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) => Promise.all(keys.filter((k) => k !== VERSION).map((k) => caches.delete(k)))).then(() => self.clients.claim())
+  );
+});
+
+self.addEventListener("fetch", (event) => {
+  const req = event.request;
+  if (req.method !== "GET") return;
+  const url = new URL(req.url);
+  if (url.origin !== self.location.origin) return;
+  if (url.pathname.startsWith("/api/")) return;
+
+  // Hashed build assets: cache-first.
+  if (url.pathname.startsWith("/assets/")) {
+    event.respondWith(
+      caches.match(req).then((hit) => hit || fetch(req).then((res) => {
+        const copy = res.clone();
+        caches.open(VERSION).then((c) => c.put(req, copy));
+        return res;
+      }))
+    );
+    return;
+  }
+
+  // Navigations & everything else: network-first, fall back to cached shell.
+  if (req.mode === "navigate") {
+    event.respondWith(fetch(req).catch(() => caches.match("/")));
+    return;
+  }
+  event.respondWith(fetch(req).catch(() => caches.match(req)));
+});