style: warmer "vintersol" treatment

A light touch of sun and warmth without overhauling the design:

1. Decorative radial sun-wash on body::before — soft peach glow from
   top-right and a warmer lemon hint from bottom-left. Pure colour
   (no image), so it doesn't trip the contrast issues the earlier
   snowflake-background hit. Dark mode gets a dimmer ember-toned
   variant so it reads as low winter sun.
2. Wordmark gains a ☀ before the title (warm, with a small text-shadow
   halo) paired with the existing ❄ after. Sun + snow makes the
   "vintersol" metaphor explicit instead of implied.
3. Primary button hover adds a ~4px sun-halo on top of the existing
   drop shadow — feels lit, not just lifted.

All within the existing palette. WCAG contrast unchanged.

frontend/src/styles.css

@@ -13,6 +13,14 @@
   --accent-fg: #ffffff;
   --danger: #b3261e;
   --card: #ffffff;
+  /* Sun-wash gradient that sits on top of --bg. A diagonal radial of
+     warm peach + lemon, very low intensity so it reads as "sunlit room"
+     not "sunset poster." Used by body::before so it stays purely
+     decorative (no contrast impact on actual content). */
+  --sun-wash:
+    radial-gradient(ellipse 70% 50% at 90% -10%, rgba(255, 196, 120, 0.35), transparent 60%),
+    radial-gradient(ellipse 60% 40% at 10% 110%, rgba(255, 230, 180, 0.20), transparent 70%);
+  --sun-glow: 255, 196, 120;   /* RGB triplet for halos; alpha set at use-site */
   /* Per-visibility colours decoupled from --accent so changing the brand
      hue doesn't accidentally make "private" look like a primary action. */
   --vis-private: #1f6feb;
@@ -41,6 +49,13 @@
     --vis-friends: #f1a528;
     --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
     --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.4);
+    /* In dark mode the sun is a distant ember rather than a noon wash —
+       lower alpha and a warmer hue so it reads as low winter sun, not
+       a spotlight. */
+    --sun-wash:
+      radial-gradient(ellipse 70% 50% at 90% -10%, rgba(251, 146, 60, 0.18), transparent 60%),
+      radial-gradient(ellipse 60% 40% at 10% 110%, rgba(220, 100, 30, 0.10), transparent 70%);
+    --sun-glow: 251, 146, 60;
   }
 }
 
@@ -55,6 +70,20 @@ html, body {
   -webkit-text-size-adjust: 100%;
 }
 
+/* Decorative sun-wash. Lives on body::before so it doesn't interact with
+   content (z-index: 0; content sits in the implicit z-index: auto stacking
+   context above). Pure colour with no image, so it doesn't trip the
+   "no decorative background image" rule that the earlier snowflake hit. */
+body::before {
+  content: '';
+  position: fixed;
+  inset: 0;
+  background: var(--sun-wash);
+  pointer-events: none;
+  z-index: 0;
+}
+main { position: relative; z-index: 1; }
+
 main {
   max-width: 720px;
   margin: 0 auto;
@@ -121,9 +150,14 @@ button.primary {
   font-weight: 600;
 }
 button.primary:hover:not(:disabled) {
-  /* Subtle elevation on the primary button — it's the most-clicked thing. */
+  /* Subtle elevation on the primary button — it's the most-clicked thing.
+     Add a warm sun-halo on top of the standard shadow so hover feels
+     "lit," not just lifted. The two shadows compose; layer-order is
+     halo first (closer to the button) then drop-shadow. */
   transform: translateY(-1px);
-  box-shadow: var(--shadow-md);
+  box-shadow:
+    0 0 0 4px rgba(var(--sun-glow), 0.18),
+    var(--shadow-md);
   background: var(--accent);
   border-color: transparent;
 }
@@ -225,14 +259,23 @@ nav.top h1 {
   position: relative;
 }
 nav.top h1 a { letter-spacing: -0.015em; }
-nav.top h1::after {
-  /* A small snowflake-like accent next to the title. */
-  content: '❄';
-  margin-left: 0.45rem;
+nav.top h1::before {
+  /* Sun before the wordmark — small, warm, glows just enough to feel lit. */
+  content: '☀';
+  margin-right: 0.35rem;
   color: var(--accent);
-  font-size: 0.85em;
+  font-size: 0.9em;
+  vertical-align: 0.02em;
+  text-shadow: 0 0 12px rgba(var(--sun-glow), 0.55);
+}
+nav.top h1::after {
+  /* Snowflake on the other side — pairs with the sun for "vintersol". */
+  content: '❄';
+  margin-left: 0.4rem;
+  color: var(--vis-private);
+  font-size: 0.8em;
   vertical-align: 0.05em;
-  opacity: 0.85;
+  opacity: 0.75;
 }
 
 /* Hero treatment on the landing — extra weight on the first paragraph so