feat(mobile): polish composer model/agent controls and selection overlay

Redesign the mobile composer model and agent buttons as borderless, full-bleed
labels that hug their content, truncate with an ellipsis when space is tight,
and show the provider logo inline before the model name. Tighten the footer
action buttons (sessions / attach / auto-accept) so they sit close together,
with a small left inset on the group. In the mobile model selection overlay,
make the thinking-variant control text-only with a chevron, vertically center
the variant and favorite controls in each row, and place the provider logo
inline with the model name.

packages/ui/src/components/chat/MobileAgentButton.tsx

@@ -73,8 +73,8 @@ export const MobileAgentButton: React.FC<MobileAgentButtonProps> = ({ onCycleAge
             onPointerLeave={handlePointerLeave}
             onContextMenu={(e) => e.preventDefault()}
             className={cn(
-                'inline-flex min-w-0 items-center select-none',
-                'rounded-lg border border-border/50 px-1.5',
+                'inline-flex min-w-0 items-stretch select-none',
+                'rounded-lg',
                 'typography-micro font-medium',
                 'focus:outline-none hover:bg-[var(--interactive-hover)]',
                 'touch-none',
@@ -88,7 +88,9 @@ export const MobileAgentButton: React.FC<MobileAgentButtonProps> = ({ onCycleAge
             }}
             title={agentLabel}
         >
-            <span className="truncate">{agentLabel}</span>
+            <span className="flex h-full w-full min-w-0 items-center">
+                <span className="truncate">{agentLabel}</span>
+            </span>
         </button>
     );
 };