feat: show remaining context window for Claude Code, Codex, and OpenCode

Track and display context window usage across all three agent providers,
letting users see how much context remains before hitting limits.

packages/app/src/components/message-input.tsx

@@ -79,6 +79,8 @@ export interface MessageInputProps {
   isInputActive?: boolean;
   /** Content to render on the left side of the button row (e.g., AgentStatusBar) */
   leftContent?: React.ReactNode;
+  /** Content to render on the right side before the voice button (e.g., context window meter) */
+  beforeVoiceContent?: React.ReactNode;
   /** Content to render on the right side after voice button (e.g., realtime button, cancel button) */
   rightContent?: React.ReactNode;
   voiceServerId?: string;
@@ -201,6 +203,7 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(funct
     disabled = false,
     isInputActive = true,
     leftContent,
+    beforeVoiceContent,
     rightContent,
     voiceServerId,
     voiceAgentId,
@@ -1015,6 +1018,7 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(funct
 
           {/* Right: voice button, contextual button (realtime/send/cancel) */}
           <View style={styles.rightButtonGroup}>
+            {beforeVoiceContent}
             <Tooltip delayDuration={0} enabledOnDesktop enabledOnMobile={false}>
               <TooltipTrigger
                 onPress={handleVoicePress}