Use the official PLM logo in the navbar and as favicon

Adds app/static/logo.svg — the official Palladium logo (fetched from the
palladium-coin/palladium-web-site repo), reprocessed for crisp display at
small sizes: the white/blue boundary is sharpened (steep contrast curve
instead of the soft anti-aliasing baked into the low-res source) and the
outer circle uses an analytically computed alpha edge instead of a
rasterized ellipse, so it stays perfectly round with no pixel staircase at
any size. Shape and colors are otherwise identical to the source artwork.

Wired up as the navbar brand mark (replacing the plain amber "P" square) in
index.html, and as the favicon for both index.html and admin.html.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/static/admin.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>PLM Lottery — Admin</title>
+<link rel="icon" type="image/svg+xml" href="/logo.svg">
 <style>
   @import url('https://fonts.googleapis.com/css2?family=Fira+Code:wght@500;600&family=Fira+Sans:wght@400;500;600;700&display=swap');
 

app/static/index.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>PLM Lottery</title>
+<link rel="icon" type="image/svg+xml" href="/logo.svg">
 <style>
   @import url('https://fonts.googleapis.com/css2?family=Fira+Code:wght@500;600&family=IBM+Plex+Sans:wght@400;500;600;700&display=swap');
 
@@ -146,12 +147,7 @@
     display: flex; align-items: center; justify-content: space-between; gap: 12px;
   }
   .app-navbar-top .brand { display: flex; align-items: center; gap: 8px; font-weight: 700; font-size: 1rem; letter-spacing: -0.01em; }
-  .app-navbar-top .brand-mark {
-    width: 26px; height: 26px; border-radius: 8px; flex-shrink: 0;
-    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
-    display: flex; align-items: center; justify-content: center;
-    color: var(--color-on-primary); font-weight: 800; font-size: 0.8rem;
-  }
+  .app-navbar-top .brand-mark { width: 26px; height: 26px; border-radius: 50%; flex-shrink: 0; display: block; }
   .app-navbar-account { display: flex; align-items: center; gap: 6px; }
   .app-navbar-account .navbar-username { display: none; font-weight: 600; font-size: 0.85rem; margin-right: 2px; }
   @media (min-width: 420px) { .app-navbar-account .navbar-username { display: inline; } }
@@ -387,7 +383,10 @@
 <nav class="hidden" id="app-navbar" aria-label="Sezioni">
   <div class="app-navbar-top">
     <div class="app-navbar-top-inner">
-      <span class="brand"><span class="brand-mark">P</span> PLM Lottery</span>
+      <span class="brand">
+        <img class="brand-mark" src="/logo.svg" alt="">
+        PLM Lottery
+      </span>
       <div class="app-navbar-account">
         <span class="navbar-username" id="dash-username"></span>
         <span class="navbar-balance mono">