feat: add extension icons and prepare for AMO submission

SVG source icon with generated PNGs at 48/96/128px. Add
browser_specific_settings with gecko ID for Firefox Add-ons.
Restrict CSP img-src to self and data URIs.

Assisted-by: Claude Code
Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

manifest.json

@@ -4,7 +4,15 @@
   "version": "0.1.0",
   "description": "A browser extension to fuzzy search tabs, history and bookmarks.",
   "icons": {
-    "48": "icons/border-48.png"
+    "48": "icons/icon-48.png",
+    "96": "icons/icon-96.png",
+    "128": "icons/icon-128.png"
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "ripple@avinal.space",
+      "strict_min_version": "109.0"
+    }
   },
   "permissions": [
     "tabs",