fix: improve accessibility — add ARIA attributes to ThemeSwitcher, ThinkingBlock, ThinkingIndicator, ErrorBoundary, SessionIcon

- ThemeSwitcher: aria-expanded, aria-haspopup, aria-pressed on theme/accent buttons, Escape to close, dialog role
- ThinkingBlock: aria-expanded on toggle, region role on content
- ThinkingIndicator: role=status, aria-label, decorative icon aria-hidden
- ErrorBoundary: role=alert on error state
- SessionIcon: aria-hidden on decorative SVG brand icons

src/components/ThinkingBlock.tsx

@@ -10,6 +10,8 @@ export function ThinkingBlock({ text }: { text: string }) {
     <div className="my-2">
       <button
         onClick={() => setOpen(!open)}
+        aria-expanded={open}
+        aria-label={t('thinking.label')}
         className="inline-flex items-center gap-1.5 rounded-2xl border border-pc-border bg-pc-elevated/35 px-3 py-1.5 text-xs text-violet-300 hover:bg-[var(--pc-hover)] transition-colors"
       >
         <Brain size={13} />
@@ -17,7 +19,7 @@ export function ThinkingBlock({ text }: { text: string }) {
         {open ? <ChevronDown size={12} className="ml-1 text-pc-text-muted" /> : <ChevronRight size={12} className="ml-1 text-pc-text-muted" />}
       </button>
       {open && (
-        <div className="mt-2 rounded-2xl border border-pc-border bg-pc-elevated/25 p-3 text-sm italic text-pc-text-secondary whitespace-pre-wrap max-h-96 overflow-y-auto">
+        <div role="region" aria-label={t('thinking.label')} className="mt-2 rounded-2xl border border-pc-border bg-pc-elevated/25 p-3 text-sm italic text-pc-text-secondary whitespace-pre-wrap max-h-96 overflow-y-auto">
           {text}
         </div>
       )}