feat(pwa): installable web app + landing web entry

Make the web SPA an installable PWA and surface it to users:

- manifest.webmanifest + an install-only service worker + 192/512/maskable
  icons (ui/public); PWA head tags in index.html; the .webmanifest MIME type
  registered in the gateway (the distroless image has no /etc/mime.types).
- Platform-adaptive install CTA (components/InstallApp.svelte + lib/pwa):
  one-tap on Chromium, manual Add-to-Home-Screen instructions on iOS Safari,
  hidden elsewhere / once installed / inside a Mini App. Shown under the
  logged-out login card and at the bottom of Settings.
- The landing gains a third entry linking /app/ (the brand tile), with a
  caption under all three (Telegram / VK / Веб-версия).

The service worker is navigation-only (network-first, cached-shell fallback);
hashed assets and the Connect stream are untouched. It exists to satisfy
Chromium's installability requirement and is the single growth point for a
future opt-in offline mode.

Tests: pwa.ts unit tests; a webui probe (manifest/sw.js content-type + the
SPA-fallback-to-HTML trap); e2e for the one-tap CTA, the iOS instructions
modal and the landing web entry. Docs: ARCHITECTURE §13, FUNCTIONAL (+ru),
gateway README.

ui/src/Landing.svelte

@@ -80,20 +80,26 @@
   <section class="hero">
     <h1>{about.title}</h1>
     <p class="tagline">{t('landing.tagline')}</p>
-    {#if tgLink || vkLink}
-      <div class="entries">
-        {#if tgLink}
-          <a class="entry" href={tgLink} target="_blank" rel="noopener noreferrer" aria-label={t('landing.playTelegram')}>
-            <img src="telegram-logo.svg" alt="" width="64" height="64" />
-          </a>
-        {/if}
-        {#if vkLink}
-          <a class="entry" href={vkLink} target="_blank" rel="noopener noreferrer" aria-label={t('landing.playVK')}>
-            <img src="vk-logo.svg" alt="" width="64" height="64" />
-          </a>
-        {/if}
-      </div>
-    {/if}
+    <!-- Platform entries: Telegram/VK are gated by their build vars; the web version (/app/) is
+         always available. A caption under each clarifies what it opens. -->
+    <div class="entries">
+      {#if tgLink}
+        <a class="entry" href={tgLink} target="_blank" rel="noopener noreferrer" aria-label={t('landing.playTelegram')}>
+          <img src="telegram-logo.svg" alt="" width="64" height="64" />
+          <span class="caption">{t('landing.captionTelegram')}</span>
+        </a>
+      {/if}
+      {#if vkLink}
+        <a class="entry" href={vkLink} target="_blank" rel="noopener noreferrer" aria-label={t('landing.playVK')}>
+          <img src="vk-logo.svg" alt="" width="64" height="64" />
+          <span class="caption">{t('landing.captionVK')}</span>
+        </a>
+      {/if}
+      <a class="entry" href="/app/" aria-label={t('landing.playWeb')}>
+        <img src="favicon.svg" alt="" width="64" height="64" />
+        <span class="caption">{t('landing.captionWeb')}</span>
+      </a>
+    </div>
   </section>
 
   <section class="info">
@@ -207,8 +213,8 @@
     color: var(--text-muted);
     font-size: 1.05rem;
   }
-  /* The platform entries are just the bigger logos in a row (no button chrome, no captions);
-     each link keeps an aria-label for assistive tech. */
+  /* The platform entries are the bigger logos in a row, each with a short caption below; the
+     link keeps an aria-label for assistive tech. */
   .entries {
     align-self: center;
     display: flex;
@@ -217,6 +223,11 @@
   }
   .entry {
     display: inline-flex;
+    flex-direction: column;
+    align-items: center;
+    gap: 8px;
+    text-decoration: none;
+    color: var(--text);
   }
   .entry img {
     display: block;
@@ -225,6 +236,10 @@
   .entry:hover img {
     transform: scale(1.06);
   }
+  .caption {
+    font-size: 0.9rem;
+    color: var(--text-muted);
+  }
   .info {
     background: var(--surface-2);
     border-radius: var(--radius-sm);