Rename 'Gatekeeper URL' to 'Node URL' in wallet settings

[macterra]

The wallet settings currently label the server connection as "Gatekeeper URL" with localStorage key gatekeeperUrl. Now that Drawbridge can serve as the entry point (and the URL could point to either gatekeeper or drawbridge), the label should be generic.

Changes needed

• Rename user-facing label from "Gatekeeper URL" to "Node URL"
• Rename localStorage key from gatekeeperUrl to nodeUrl (with migration for existing users)
• Rename constants: GATEKEEPER_KEY → NODE_URL_KEY, DEFAULT_GATEKEEPER_URL → DEFAULT_NODE_URL
• Rename env var: VITE_GATEKEEPER_URL → VITE_NODE_URL
• Update internal variable names across react-wallet, browser-extension, gatekeeper-client, keymaster-client

Affected files

• apps/react-wallet/src/constants.ts
• apps/react-wallet/src/components/SettingsTab.tsx
• apps/react-wallet/src/components/FileTab.tsx
• apps/react-wallet/src/components/ImageTab.tsx
• apps/react-wallet/src/contexts/WalletProvider.tsx
• apps/gatekeeper-client/src/App.jsx
• apps/gatekeeper-client/src/KeymasterUI.jsx
• apps/keymaster-client/src/App.jsx
• apps/keymaster-client/src/KeymasterUI.jsx
• apps/browser-extension/src/components/SettingsTab.tsx
• apps/browser-extension/src/components/FileTab.tsx
• apps/browser-extension/src/components/ImageTab.tsx