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/exportChat.ts

@@ -0,0 +1,79 @@
+import type { ChatMessage } from '../types';
+
+/**
+ * Convert a list of chat messages into a Markdown string suitable for export.
+ */
+export function messagesToMarkdown(messages: ChatMessage[], sessionLabel?: string): string {
+  const lines: string[] = [];
+
+  if (sessionLabel) {
+    lines.push(`# ${sessionLabel}`, '');
+  }
+
+  const exportDate = new Date().toISOString().replace('T', ' ').replace(/\.\d+Z$/, ' UTC');
+  lines.push(`> Exported from PinchChat on ${exportDate}`, '');
+
+  for (const msg of messages) {
+    const time = new Date(msg.timestamp).toLocaleString('en-US', {
+      dateStyle: 'medium',
+      timeStyle: 'short',
+    });
+
+    const roleLabel = msg.role === 'user'
+      ? (msg.isSystemEvent ? '⚙️ System Event' : '👤 User')
+      : '🤖 Assistant';
+
+    lines.push(`## ${roleLabel} — ${time}`, '');
+
+    if (msg.blocks && msg.blocks.length > 0) {
+      for (const block of msg.blocks) {
+        switch (block.type) {
+          case 'text':
+            lines.push(block.text, '');
+            break;
+          case 'thinking':
+            lines.push('<details><summary>💭 Thinking</summary>', '', block.text, '', '</details>', '');
+            break;
+          case 'tool_use':
+            lines.push(`**🔧 Tool: \`${block.name}\`**`, '');
+            if (block.input && Object.keys(block.input).length > 0) {
+              lines.push('```json', JSON.stringify(block.input, null, 2), '```', '');
+            }
+            break;
+          case 'tool_result':
+            if (block.content) {
+              const label = block.name ? `Result (${block.name})` : 'Result';
+              lines.push(`<details><summary>📋 ${label}</summary>`, '');
+              lines.push('```', block.content.slice(0, 5000), '```', '');
+              lines.push('</details>', '');
+            }
+            break;
+          case 'image':
+            lines.push('*[Image]*', '');
+            break;
+        }
+      }
+    } else if (msg.content) {
+      lines.push(msg.content, '');
+    }
+
+    lines.push('---', '');
+  }
+
+  return lines.join('\n');
+}
+
+/**
+ * Trigger a browser file download with the given content.
+ */
+export function downloadFile(content: string, filename: string, mimeType = 'text/markdown') {
+  const blob = new Blob([content], { type: mimeType });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}

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;