i18n: src/extensions/core/ has 20+ hardcoded English strings (groupNode, nodeTemplates, electronAdapter, etc.)

Multiple extension files under `src/extensions/core/` contain hardcoded English strings that do not use i18n.

### File Details

**groupNode.ts + groupOptions.ts + groupNodeManage.ts**
- "An in use group node with this name already exists..." — toast
- "A group node with this name already exists..." — confirm dialog
- "Remove from workflow", "Removed", "Convert to nodes", "Manage Group Node"
- "Running {n.title || n.type}..."
- "Inputs", "Widgets", "Outputs" (tab names), "Visible"
- "This group node is in use..."
- 12+ context menu items ("Add Group For Selected Nodes", "Fit Group To Nodes", "Select Nodes", "Bypass Group Nodes", etc.)

**nodeTemplates.ts**
- "Close", "Export", "Import", "Name:", "Delete"
- "Save Selected as Template", "Manage", "Node Templates"

**electronAdapter.ts** — 22 setting names and tooltips hardcoded:
- "Automatically check for updates", "Send anonymous usage metrics"
- "Window Style" + tooltip "Custom: Replace the system title bar..."
- "Python Install Mirror", "Pypi Install Mirror", "Torch Install Mirror" + tooltips
- Commands: "Open Logs Folder", "Open Models Folder", "Open Outputs Folder", "Open Inputs Folder", "Open Custom Nodes Folder", "Open extra_model_paths.yaml", "Open DevTools", "Desktop User Guide", "Check for Updates"
- "Reinstall", "Restart", "Quit"

**Other files:**
- editAttention.ts: "Ctrl+up/down precision"
- clipspace.ts: "Close"
- contextMenuFilter.ts: "Filter list"
- webcamCapture.ts: error toasts use `err` directly

### Suggested Fix

Add these strings to `src/locales/en/main.json` under an extensions namespace, and replace hardcoded strings with `t()` calls.