"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/static/manifest.webmanifest

@@ -5,9 +5,12 @@
   "start_url": "/",
   "scope": "/",
   "display": "standalone",
-  "background_color": "#faf6ee",
-  "theme_color": "#2f7d5b",
+  "background_color": "#f7f4ec",
+  "theme_color": "#0083ad",
   "icons": [
-    { "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any maskable" }
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" },
+    { "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" }
   ]
 }