fix(plating): chatter action toolbar invisible in dark mode

The floating message-action toolbar (reaction / reply / star / link
icons) appearing on hover renders white-icons-on-white-background in
dark mode — Odoo's own dark.scss sets the icon hover color to white
but never gives the toolbar itself a dark background. Result: the
icons vanish entirely in dark mode.

Add fp_chatter_dark.scss that branches at compile time on
$o-webclient-color-scheme == dark (Odoo 19 compiles every SCSS file
into both web.assets_backend with `bright` AND web.assets_web_dark
with `dark`) and gives the toolbar:

- Solid dark background (#2b2f33 fallback, var(--o-component-bgcolor))
- Subtle 1px white-alpha border + drop shadow so it floats nicely
- Icon color rgba(255,255,255,.78) at full opacity (not 35%)
- Brighter hover state with a subtle bg highlight

Light bundle output is empty (the @if branch doesn't fire), so the
light theme is untouched.

Verified: dark bundle includes our rule with #2b2f33 marker present.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/scripts/fp_dark_bundle_check.py

@@ -0,0 +1,31 @@
+env = env  # noqa
+# Force generation of both bundles
+for bundle_name in ('web.assets_backend', 'web.assets_web_dark'):
+    bundle = env['ir.qweb']._get_asset_bundle(bundle_name)
+    css = bundle.css()  # this materializes the attachment
+    print(f'{bundle_name}: triggered, css() type={type(css).__name__}')
+
+env.cr.commit()
+
+# Now find them
+attachs = env['ir.attachment'].sudo().search(
+    [('url', 'like', '/web/assets/%')],
+    order='id desc',
+)
+print(f'\\n{len(attachs)} asset attachments after force-compile:')
+for a in attachs:
+    raw_size = len(a.raw or b'')
+    print(f'  [{a.id}] {a.name} ({raw_size} bytes)')
+
+# Check the dark one for our marker
+dark = attachs.filtered(lambda a: 'web.assets_web_dark' in (a.name or ''))
+if dark:
+    text = (dark[0].raw or b'').decode('utf-8', errors='ignore')
+    print(f'\\ndark bundle markers:')
+    print(f'  o-mail-Message-actions: {text.count("o-mail-Message-actions")} occurrences')
+    print(f'  #2b2f33 marker        : {text.count("#2b2f33")} occurrences')
+    print(f'  rgba(255, 255, 255, 0.10) marker: {text.count("rgba(255, 255, 255, 0.10)")} occurrences')
+    if '#2b2f33' in text:
+        idx = text.find('#2b2f33')
+        print(f'\\ncontext around our color:')
+        print(text[max(0, idx-300):idx+300])