feat: non-blocking batch capture dialog, toasts on failure, fix /runs errors

CaptureDialog:
- Replace single textarea with multi-row inputs; + button adds rows
- Submit fires all pending rows in parallel, dialog stays open/usable
- Polling intervals live on a persistent ref (not cleared on close) so
  toasts fire even after the dialog is dismissed
- archiveId stored per item at submit time; page-refresh reconnect uses
  it.archiveId instead of the possibly-null prop
- Completed rows flash green then self-remove; failed rows show inline
  error + retry button
- Cancel becomes Close while jobs are in flight

ToastStack (new component):
- Fixed bottom-right overlay with spring-in animation
- Error toast: truncated locator, View error / Hide toggle expanding
  full error_text in a monospace pre block
- Auto-dismisses after 7 s; timer pauses while detail is expanded

RunsView:
- Failed rows are clickable and expand a full-width detail row showing
  error_summary in a scrollable monospace block

capture.rs (archivr-core):
- Staging dir is now "{millis}-{uuid}" — parallel captures in the same
  millisecond can no longer collide on temp paths
- create_archive_run moved before URL Content-Type probe so every
  attempt appears in /runs regardless of outcome
- Probe failures now call create_archive_run_item with source_metadata
  fallback then fail_run, recording error_text on the item and
  error_summary on the run with correct failed_count

styles.css:
- Capture dialog: header row, multi-row layout, status dots, spinner,
  add-row dashed button, per-row error text
- Toast stack: fixed overlay, error card with coloured left border,
  monospace detail expansion
- Run error rows: clickable hover tint, expand hint chevron, detail pre

frontend/src/components/ToastStack.jsx

@@ -0,0 +1,63 @@
+import { useState, useEffect } from 'react'
+
+const TOAST_TTL = 7000 // ms before auto-dismiss; paused when error is expanded
+
+export default function ToastStack({ toasts, onDismiss }) {
+  if (!toasts.length) return null
+  return (
+    <div className="toast-stack" role="log" aria-live="polite" aria-label="Notifications">
+      {toasts.map(t => (
+        <Toast key={t.id} toast={t} onDismiss={onDismiss} />
+      ))}
+    </div>
+  )
+}
+
+function Toast({ toast, onDismiss }) {
+  const [expanded, setExpanded] = useState(false)
+
+  // Auto-dismiss after TTL; paused while error detail is expanded
+  useEffect(() => {
+    if (expanded) return
+    const timer = setTimeout(() => onDismiss(toast.id), TOAST_TTL)
+    return () => clearTimeout(timer)
+  }, [expanded, toast.id, onDismiss])
+
+  const short = toast.locator
+    ? (toast.locator.length > 48 ? toast.locator.slice(0, 45) + '\u2026' : toast.locator)
+    : null
+
+  return (
+    <div className="toast toast--error" role="alert" aria-atomic="true">
+      <div className="toast-top">
+        <span className="toast-icon" aria-hidden="true">✕</span>
+        <div className="toast-body">
+          <span className="toast-headline">Capture failed</span>
+          {short && <span className="toast-locator">{short}</span>}
+        </div>
+        <div className="toast-btns">
+          {toast.errorText && (
+            <button
+              type="button"
+              className="toast-view-btn"
+              onClick={() => setExpanded(v => !v)}
+            >
+              {expanded ? 'Hide' : 'View error'}
+            </button>
+          )}
+          <button
+            type="button"
+            className="toast-dismiss"
+            onClick={() => onDismiss(toast.id)}
+            aria-label="Dismiss"
+          >
+            ×
+          </button>
+        </div>
+      </div>
+      {expanded && toast.errorText && (
+        <pre className="toast-error-detail">{toast.errorText}</pre>
+      )}
+    </div>
+  )
+}