feat(icons): wire extension and toolbar icons; add icons/README with placement and size guidance

icons/README.md

@@ -0,0 +1,27 @@
+Icon assets
+===========
+
+Place the final PNGs for the add-on icon here. The manifest is wired to use:
+
+- Toolbar (browser_action → default_icon):
+  - icons/icon-16.png
+  - icons/icon-32.png
+- Extension/Store (top-level icons):
+  - icons/icon-48.png
+  - icons/icon-96.png
+  - icons/icon-128.png
+
+Notes
+-----
+- Use the same artwork at each size; keep transparent backgrounds.
+- Source: a 128×128 PNG is perfect; export downscaled versions for 96/48/32/16.
+- Firefox does not require .ico; PNG is recommended.
+- If you later want theme-specific toolbar icons, you can add `browser_action.theme_icons` with light/dark variants.
+
+Quick export (ImageMagick)
+-------------------------
+magick icon-128.png -resize 96x96   icon-96.png
+magick icon-128.png -resize 48x48   icon-48.png
+magick icon-128.png -resize 32x32   icon-32.png
+magick icon-128.png -resize 16x16   icon-16.png
+