fix(app): balance composer right-side button spacing

Tightens transparent icon gaps to 4px and gives filled action buttons
(send, interrupt) a 4px left margin so they sit 8px from neighbors.
Shrinks context-window meter SVG from 20 to 16 to match the visual
weight of surrounding icons.

packages/app/src/components/context-window-meter.tsx

@@ -8,7 +8,7 @@ type ContextWindowMeterProps = {
   usedTokens: number;
 };
 
-const SVG_SIZE = 20;
+const SVG_SIZE = 16;
 const CENTER = SVG_SIZE / 2;
 const RADIUS = 7;
 const STROKE_WIDTH = 2.25;