fix(briefing): responsive sidebar, remove dividers, scale forecast icons

Switch sidebar from fixed px to proportional 35% so it resizes with
the viewport. Remove border between chat and input bar and between
weather and news. Use container query units for weather/forecast icons
so they scale with column width.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/WeatherCard.vue

@@ -123,6 +123,7 @@ const fetchedAtLabel = computed(() => {
   padding: 1rem 1.25rem;
   margin-bottom: 1rem;
   font-size: 0.9rem;
+  container-type: inline-size;
 }
 
 .weather-header {
@@ -150,12 +151,12 @@ const fetchedAtLabel = computed(() => {
 }
 
 .weather-icon {
-  font-size: 2rem;
+  font-size: clamp(1.5rem, 5cqi, 2.5rem);
   line-height: 1;
 }
 
 .weather-temp {
-  font-size: 2rem;
+  font-size: clamp(1.5rem, 5cqi, 2.5rem);
   font-weight: 700;
   line-height: 1;
 }
@@ -209,7 +210,7 @@ const fetchedAtLabel = computed(() => {
 }
 
 .forecast-icon {
-  font-size: 1.1rem;
+  font-size: clamp(0.9rem, 3cqi, 1.3rem);
   line-height: 1;
 }