efactor(extension): modularize dashboard, background, and docs

Split monolithic dashboard.js (2753 lines) into 18 focused modules
under dashboard/{core,data,ui,pages}/ with refresh.js and events.js
as orchestrators. Extracted ~900-line inline <style> into dashboard.css
and moved dashboard.html to dashboard/dashboard.html.

Split background.js (609 lines) into background/{logs,state,proxy,
native-messaging,tab-lifecycle,message-router}.js with a thin entry
point using importScripts().

Deleted dead files: wrong-domain.js (duplicate of inline script).
Updated manifest.json web_accessible_resources for new paths.
Updated docs/ARCHITECTURE.md to reflect the new file structure.

No functionality changed. No build step introduced.

extension/manifest.json

@@ -42,7 +42,16 @@
         "vendor/xterm-addon-fit.js",
         "vendor/xterm.css",
         "vendor/novnc.js",
-        "wrong-domain.html"
+        "wrong-domain.html",
+        "dashboard/dashboard.html",
+        "dashboard/dashboard.css",
+        "dashboard/data/*.js",
+        "dashboard/core/*.js",
+        "dashboard/ui/*.js",
+        "dashboard/pages/*.js",
+        "dashboard/refresh.js",
+        "dashboard/events.js",
+        "background/*.js"
       ],
       "matches": [
         "chrome-extension://*/*"