feat: improve accessibility — semantic HTML landmarks, skip-to-content link, ARIA attributes

- Replace div with <main> for primary chat pane (screen reader landmark)
- Replace div with <section> for split pane
- Add skip-to-content link for keyboard navigation (Tab → jump to chat input)
- Add aria-expanded and aria-label to tool call badge buttons
- Add id='chat-input' to textarea for skip link target
- Add i18n keys for new ARIA labels (EN + FR)

src/components/ChatInput.tsx

@@ -304,6 +304,7 @@ export function ChatInput({ onSend, onAbort, isGenerating, disabled, sessionKey
             />
 
             <HighlightedTextarea
+              id="chat-input"
               ref={textareaRef}
               value={text}
               highlightEnabled={highlightEnabled}