Version the brand images' URLs, so a new mark reaches returning visitors

The logo, favicons and app icons are served from public/img under fixed
names with a browser cache of hours, and the service worker fetches them
through that cache. After the mark changed on 2026-09-27, returning
visitors kept the old cat until their copies expired, and the favicon
and an installed app's icon hold on longer still.

Every URL that names one now carries ?v=BRAND_V (src/lib/brand.ts,
brandImage()): the header, sign-in, About, the mail empty state, the
notification icons, index.html's favicon links, the manifest's icons
and the service worker's shell and notification icons. Date-stamped,
never a counter, for the sites' ASSET_V reason; the three static files
carry the value written out, and the comment says to keep them in step.

web/public/manifest.webmanifest

@@ -51,17 +51,17 @@
   "theme_color": "#0f766e",
   "icons": [
     {
-      "src": "img/icon-192.png",
+      "src": "img/icon-192.png?v=2026-09-27a",
       "sizes": "192x192",
       "type": "image/png"
     },
     {
-      "src": "img/icon-512.png",
+      "src": "img/icon-512.png?v=2026-09-27a",
       "sizes": "512x512",
       "type": "image/png"
     },
     {
-      "src": "img/icon-maskable.png",
+      "src": "img/icon-maskable.png?v=2026-09-27a",
       "sizes": "192x192",
       "type": "image/png",
       "purpose": "maskable"