feat: export conversation as Markdown file

Add a download button in the header that exports the current session's
messages as a well-formatted Markdown file. Includes:
- User/assistant/system event labels with timestamps
- Thinking blocks as collapsible <details>
- Tool calls with JSON parameters
- Tool results as collapsible sections
- Image placeholders
- Session label and export date in header

i18n: EN + FR translations for the export tooltip.

src/lib/i18n.ts

@@ -94,6 +94,9 @@ const en = {
   'error.reload': 'Reload page',
   'shortcuts.navigationSection': 'Navigation',
   'shortcuts.generalSection': 'General',
+
+  // Export
+  'header.export': 'Export conversation as Markdown',
 } as const;
 
 const fr: Record<keyof typeof en, string> = {
@@ -172,6 +175,8 @@ const fr: Record<keyof typeof en, string> = {
   'error.reload': 'Recharger',
   'shortcuts.navigationSection': 'Navigation',
   'shortcuts.generalSection': 'Général',
+
+  'header.export': 'Exporter la conversation en Markdown',
 };
 
 export type TranslationKey = keyof typeof en;