feat: SIWE auth, auto-pin, pin search/share, explorer improvements

- SIWE server-side auth (siwe + jose JWT)
- Wallet provider fallback (MAOS -> MetaMask)
- Auto-pin after upload (quick + crypto)
- Pin search/filter in explorer
- Share button (gateway link) per pin
- Copy CID button per pin
- Breadcrumb root navigation fix
- Hydration mismatch fix (mounted state)
- Defensive truncateCid for empty values
- CSP fix (template literal)
- Error boundaries (upload, profile, usage)
- 191 tests passing

package.json

@@ -19,6 +19,7 @@
     "postcss": "^8.5.15",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
+    "siwe": "^3.0.0",
     "swr": "^2.4.2",
     "viem": "^2.29.0"
   },