Add campaign layer, GPU model caching, file browser cleanup

- Campaigns: new table, CRUD API, React context + provider
- Sessions: scoped to campaigns, paths under campaigns/{id}/
- File browser: scoped per campaign, removed copy/paste/autoPlay
- Sidebar: campaign selector dropdown at top
- Transcribe: GPU model cached/released via job counter
- Jobs: status text updates dynamically in real-time
- Auto-redirect: blocked when summarize job is active

frontend/src/contexts/CampaignContext.tsx

@@ -0,0 +1,59 @@
+import { createContext, useContext, useState, useEffect, useCallback, type ReactNode } from 'react';
+import { campaignApi, type Campaign } from '../api';
+
+interface CampaignContextType {
+  currentCampaign: Campaign | null;
+  setCampaign: (c: Campaign | null) => void;
+  campaigns: Campaign[];
+  refreshCampaigns: () => void;
+}
+
+const CampaignContext = createContext<CampaignContextType | null>(null);
+
+export function CampaignProvider({ children }: { children: ReactNode }) {
+  const [campaigns, setCampaigns] = useState<Campaign[]>([]);
+  const [currentCampaign, setCurrentCampaign] = useState<Campaign | null>(null);
+
+  const refreshCampaigns = useCallback(async () => {
+    try {
+      const list = await campaignApi.list();
+      setCampaigns(list);
+    } catch {}
+  }, []);
+
+  useEffect(() => {
+    refreshCampaigns();
+  }, [refreshCampaigns]);
+
+  useEffect(() => {
+    if (campaigns.length === 0) return;
+    if (!currentCampaign) {
+      const stored = localStorage.getItem('campaignId');
+      const match = stored ? campaigns.find(c => c.id === stored) : null;
+      setCurrentCampaign(match || campaigns[0]);
+    } else {
+      const stillExists = campaigns.find(c => c.id === currentCampaign.id);
+      if (!stillExists) {
+        setCurrentCampaign(campaigns[0] || null);
+      }
+    }
+  }, [campaigns]);
+
+  const setCampaign = (c: Campaign | null) => {
+    if (c) localStorage.setItem('campaignId', c.id);
+    else localStorage.removeItem('campaignId');
+    setCurrentCampaign(c);
+  };
+
+  return (
+    <CampaignContext.Provider value={{ currentCampaign, setCampaign, campaigns, refreshCampaigns }}>
+      {children}
+    </CampaignContext.Provider>
+  );
+}
+
+export function useCampaign() {
+  const ctx = useContext(CampaignContext);
+  if (!ctx) throw new Error('useCampaign must be used within CampaignProvider');
+  return ctx;
+}