feat: sidebar CSS inspector — element picker, box model, rule cascade, quick edit

Extension changes for the visual CSS inspector:
- inspector.js: element picker with hover highlight, CSS selector generation,
  basic mode fallback (getComputedStyle + CSSOM), page alteration handlers
- inspector.css: picker overlay styles (blue highlight + tooltip)
- background.js: inspector message routing (picker <-> server <-> sidepanel)
- sidepanel: Inspector tab with box model viz (gstack palette), matched rules
  with specificity badges, computed styles, click-to-edit quick edit,
  Send to Agent/Code button, empty/loading/error states

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/manifest.json

@@ -3,7 +3,7 @@
   "name": "gstack browse",
   "version": "0.1.0",
   "description": "Live activity feed and @ref overlays for gstack browse",
-  "permissions": ["sidePanel", "storage", "activeTab"],
+  "permissions": ["sidePanel", "storage", "activeTab", "scripting"],
   "host_permissions": ["http://127.0.0.1:*/"],
   "action": {
     "default_icon": {