feat: add session resume capability with automatic continuation

Add ability to resume inactive sessions and automatically continue with new
session ID. Includes message merging for preserving conversation history,
UI improvements to allow sending during inactive state, and database schema
migration. close #87

web/src/api/client.ts

@@ -253,6 +253,14 @@ export class ApiClient {
         })
     }
 
+    async resumeSession(sessionId: string): Promise<string> {
+        const response = await this.request<{ sessionId: string }>(
+            `/api/sessions/${encodeURIComponent(sessionId)}/resume`,
+            { method: 'POST' }
+        )
+        return response.sessionId
+    }
+
     async sendMessage(sessionId: string, text: string, localId?: string | null, attachments?: AttachmentMetadata[]): Promise<void> {
         await this.request(`/api/sessions/${encodeURIComponent(sessionId)}/messages`, {
             method: 'POST',