feat(a11y): label icon-only buttons and inputs for screen readers

Header, message action, send/stop, history, model selector and dark-mode controls had no accessible name (icon-only buttons). Adds roles/labels and selected/disabled state; no visual change.

__tests__/ChatInput.test.tsx

@@ -0,0 +1,39 @@
+import { fireEvent, render, screen } from '@testing-library/react-native';
+import React from 'react';
+import { ChatInput } from '../components/chat/ChatInput';
+
+describe('ChatInput', () => {
+    it('sends trimmed text, clears the field and disables send while empty', () => {
+        const onSend = jest.fn();
+        render(<ChatInput onSend={onSend} />);
+
+        expect(screen.getByLabelText('Send message').props.accessibilityState.disabled).toBe(true);
+
+        fireEvent.changeText(screen.getByLabelText('Message'), '  hello  ');
+        fireEvent.press(screen.getByLabelText('Send message'));
+
+        expect(onSend).toHaveBeenCalledWith('hello');
+        expect(screen.getByLabelText('Message').props.value).toBe('');
+    });
+
+    it('shows a stop button instead of send while loading', () => {
+        const onStop = jest.fn();
+        render(<ChatInput onSend={jest.fn()} onStop={onStop} isLoading />);
+
+        expect(screen.queryByLabelText('Send message')).toBeNull();
+        fireEvent.press(screen.getByLabelText('Stop generating'));
+        expect(onStop).toHaveBeenCalledTimes(1);
+    });
+
+    it('fills the field when editing and lets the user cancel', () => {
+        const onCancelEdit = jest.fn();
+        render(<ChatInput onSend={jest.fn()} editingText="old text" onCancelEdit={onCancelEdit} />);
+
+        expect(screen.getByText('Editing message')).toBeTruthy();
+        expect(screen.getByLabelText('Message').props.value).toBe('old text');
+
+        fireEvent.press(screen.getByLabelText('Cancel editing'));
+        expect(onCancelEdit).toHaveBeenCalledTimes(1);
+        expect(screen.getByLabelText('Message').props.value).toBe('');
+    });
+});