fix: SSTV location settings and panel sizing

- Fix GPS button not working (pass button element to handler)
- Hide output element in SSTV mode to allow panels to fill space
- Add explicit height rules for SSTV panels to expand vertically

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/css/modes/sstv.css

@@ -20,6 +20,7 @@
     padding: 16px;
     min-height: 0;
     flex: 1;
+    height: 100%;
     overflow: hidden;
 }
 
@@ -30,8 +31,9 @@
     display: flex;
     flex-direction: row;
     gap: 16px;
-    flex: 1;
-    min-height: 0;
+    flex: 1 1 auto;
+    min-height: 400px;
+    height: 100%;
     overflow: hidden;
 }