Add job error persistence and friendly error UI

Backend:
- Migration 0003_job_error: adds error_message and error_code columns to jobs table
- models/job.go: add ErrorMessage, ErrorCode fields to Job struct; SetError method; update all SELECT queries
- models/job_log.go: GetAllFiltered also reads error_message and error_code (via Job embed)
- syncengine/engine.go: setJobError() helper; capture errors at Wol timeout (wol_timeout), rsync error (rsync_error), and exit_code failure points
- api/dto.go: add ErrorMessage and ErrorCode to JobResponse
- api/handlers_jobs.go: jobToResp propagates error fields

Frontend:
- api/client.ts: add error_message? and error_code? to Job interface
- lib/status.ts: add ERROR_CODES map with friendly titles/hints; getErrorCodeInfo()
- components/ErrorDetailsModal.tsx: new modal showing error title, hint, full message, job metadata, and stderr log; copy-all and download-log buttons
- pages/JobDetail.tsx: error banner for failed jobs with title/hint; View error button opens ErrorDetailsModal; SSE updates error_message in real-time

web/src/lib/status.ts

@@ -54,3 +54,36 @@ export function statusLabel(status: string): string {
   }
   return labels[status.toLowerCase()] ?? status
 }
+
+export interface ErrorCodeInfo {
+  title: string;
+  hint: string;
+}
+
+const ERROR_CODES: Record<string, ErrorCodeInfo> = {
+  wol_timeout: {
+    title: "Machine didn't wake up",
+    hint: 'Check Wake-on-LAN settings, MAC address, and network connectivity',
+  },
+  rsync_error: {
+    title: 'rsync failed',
+    hint: 'See the full log for rsync error details',
+  },
+  exit_code: {
+    title: 'rsync exited with errors',
+    hint: 'Check stderr output for details',
+  },
+  ssh_key_fallback: {
+    title: 'SSH key not found',
+    hint: 'Server fell back to its own key. Verify the machine SSH key configuration',
+  },
+  cancelled: {
+    title: 'Cancelled by user',
+    hint: 'The job was manually cancelled',
+  },
+}
+
+export function getErrorCodeInfo(code: string | null | undefined): ErrorCodeInfo | null {
+  if (!code) return null
+  return ERROR_CODES[code] ?? { title: code, hint: 'See full error details below' }
+}