"Since you last visited" cue + PWA install (add to home screen)

Two calm returning-reader features.

Since-last-visit (Highlights companion, not a nav lane — per Codex):
* queries.feed gains a `since` filter; GET /api/since?ts= returns the count +
  a few accepted/non-dup/visible articles discovered since the reader's last
  visit (boundary-respecting; invalid/future ts → 0, no error).
* Home stores last_seen in localStorage (reads prev, then stamps now); on
  Highlights, a gentle "Since you were last here, N new calm reads came in"
  note with a "See what's new" reveal of a compact inline section. Dismissible.
  No badges, no unread counts, no "missed" language.

PWA:
* Real PNG icons (192/512 + full-bleed maskable) rasterized from favicon.svg;
  manifest fixed (azure theme to match the brand, PNG icons); apple-touch-icon.
* Minimal service worker: precache the app shell, always-fresh API + /a/ pages.
* Gentle, dismissible install banner (beforeinstallprompt → Install; iOS → the
  Share → Add to Home Screen hint). Never nags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/service-worker.js

@@ -0,0 +1,40 @@
+/// <reference types="@sveltejs/kit" />
+// Minimal, calm service worker: precache the app shell + static assets so the
+// site is installable (PWA) and the shell opens fast / works offline. Live data
+// (the API and server-rendered /a/ pages) is always fetched fresh, never staled.
+import { build, files, version } from '$service-worker';
+
+const CACHE = `upbeat-${version}`;
+const SHELL = [...build, ...files];
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(caches.open(CACHE).then((c) => c.addAll(SHELL)).then(() => self.skipWaiting()));
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches
+      .keys()
+      .then((keys) => Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k))))
+      .then(() => self.clients.claim())
+  );
+});
+
+self.addEventListener('fetch', (event) => {
+  const { request } = event;
+  if (request.method !== 'GET') return;
+  const url = new URL(request.url);
+  if (url.origin !== location.origin) return;
+
+  // Always-fresh, never cached: the API and the server-rendered article pages.
+  if (url.pathname.startsWith('/api/') || url.pathname.startsWith('/a/')) return;
+
+  // Navigations: serve the cached app shell when offline (SPA fallback).
+  if (request.mode === 'navigate') {
+    event.respondWith(fetch(request).catch(() => caches.match('/') || caches.match('/index.html')));
+    return;
+  }
+
+  // Static assets (JS/CSS/icons/fonts): cache-first, fall back to network.
+  event.respondWith(caches.match(request).then((cached) => cached || fetch(request)));
+});