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/App.jsx

@@ -13,6 +13,7 @@ import CollectionsView from './components/CollectionsView'
 import SettingsView from './components/SettingsView'
 import ContextRail from './components/ContextRail'
 import { displayPath } from './utils'
+import ToastStack from './components/ToastStack'
 
 export const AuthContext = createContext(null);
 
@@ -82,6 +83,9 @@ export default function App() {
     return saved === 'true'
   })
 
+  const [toasts, setToasts] = useState([])
+  const toastIdRef = useRef(0)
+
   const humanizeTags = currentUser?.humanize_slugs ?? false;
 
   // Persist captureDialogOpen to sessionStorage
@@ -234,6 +238,15 @@ export default function App() {
     ])
   }, [archiveId, searchQuery, tagFilter, loadEntries])
 
+  const handleToast = useCallback((errorText, locator) => {
+    const id = ++toastIdRef.current
+    setToasts(prev => [...prev, { id, errorText, locator }])
+  }, [])
+
+  const handleDismissToast = useCallback((id) => {
+    setToasts(prev => prev.filter(t => t.id !== id))
+  }, [])
+
   if (authState === 'loading') return <div className="auth-loading">Loading\u2026</div>;
   if (authState === 'setup')   return <SetupPage onComplete={() => setAuthState('login')} />;
   if (authState === 'login')   return <LoginPage onLogin={user => { setCurrentUser(user); setAuthState('authenticated'); }} />;
@@ -332,7 +345,9 @@ export default function App() {
           archiveId={archiveId}
           onClose={handleCaptureClose}
           onCaptured={handleCaptured}
+          onToast={handleToast}
         />
+        <ToastStack toasts={toasts} onDismiss={handleDismissToast} />
       </>
     </AuthContext.Provider>
   )