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/views/BriefingView.vue

@@ -354,7 +354,7 @@ onMounted(async () => {
 
 .briefing-shell {
   display: grid;
-  grid-template-columns: 1fr minmax(420px, 620px);
+  grid-template-columns: 1fr minmax(320px, 35%);
   grid-template-rows: auto 1fr;
   height: 100%;
   min-height: 0;
@@ -442,6 +442,10 @@ onMounted(async () => {
   min-height: 0;
 }
 
+.briefing-chat-panel :deep(.input-wrapper) {
+  border-top: none;
+}
+
 /* ─── Right column (Weather + News) ──────────────────────────────────────── */
 
 .briefing-right {
@@ -456,7 +460,6 @@ onMounted(async () => {
 .weather-section {
   flex-shrink: 0;
   padding: 1rem 1rem 0.5rem;
-  border-bottom: 1px solid var(--color-border);
 }
 
 .weather-section :deep(.weather-card) {