Add accessibility landmarks and ARIA attributes

[ryuhmanov-m]

Parent issue: #21

Context

The app lacks semantic HTML landmarks, making it difficult for screen readers and keyboard-only users to navigate.

Scope

1. Add landmark roles to layout:
   • <main> for the chat/content area
   • <nav> for the sidebar
   • <aside> for panels (history, knowledge)
   • <header> for the top bar
2. Add role="treeitem" to sidebar tree items (conversations, files)
3. Wrap settings sections in <fieldset> with <legend>
4. Add skip navigation link (hidden, visible on focus): "Skip to chat input"
5. Ensure focus order follows visual order

References

• Layout: src/app/layout.tsx, src/features/layout/
• Sidebar: src/components/sidebar/Sidebar.tsx
• Settings: src/components/settings/SettingsPanel.tsx