Collapse WhatsApp Web to a Signal-style avatar rail in slim windows (#6473)

* Make menu overflow visible with a half-row peek and edge fades

Menus used to cut off clean at ten rows, so anything below the fold was
undiscoverable. Now the list sizes to what fits within 60% of the screen
and always ends mid-row when items overflow, with scroll-position-driven
fades at both edges. Keyboard navigation keeps the next hidden row peeking
past the cursor so the fold affordance travels with the selection.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Collapse WhatsApp Web to a Signal-style avatar rail in slim windows

Add a WhatsApp Slim Chromium extension that injects CSS into
web.whatsapp.com, collapsing the chat list to a 90px avatar rail with
floating unread badges when the window is narrower than 1100px. Wide
windows keep the stock layout.

Loaded through the existing --load-extension list in chromium-flags.conf,
with a migration to append it for existing users.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

config/chromium-flags.conf

@@ -2,4 +2,4 @@
 --ozone-platform-hint=wayland
 --password-store=gnome-libsecret
 --enable-features=TouchpadOverscrollHistoryNavigation
---load-extension=/usr/share/omarchy/default/chromium/extensions/copy-url,/usr/share/omarchy/default/chromium/extensions/yt-dlp
+--load-extension=/usr/share/omarchy/default/chromium/extensions/copy-url,/usr/share/omarchy/default/chromium/extensions/yt-dlp,/usr/share/omarchy/default/chromium/extensions/whatsapp-slim