feat(portal-shell): ghost-style Sign-in button with log-in icon

Header's anonymous CTA was a filled brand-primary block — visually
loud against the otherwise quiet header icon strip (Notifications,
Help). Switched to a ghost (transparent / no border) button with a
log-in icon ahead of the label:

- bg-transparent, text in brand-primary-500 (dark mode: -300)
- hover surfaces a faint brand-primary-50 fill so the affordance
  stays readable
- gap-1.5 between the new 16 px log-in icon and the label
- h-11 preserved — 44x44 touch target per ADR-0016 still holds

Adds 'log-in' (lucide LogIn) to the shared icon registry; pair
of the existing 'log-out'.

libs/shared/ui/src/lib/icon/icon.ts

@@ -15,6 +15,7 @@ import {
   Grid2x2,
   House,
   LayoutDashboard,
+  LogIn,
   LogOut,
   type LucideIconData,
   LucideAngularModule,
@@ -67,6 +68,7 @@ const ICON_REGISTRY = {
   'grid-2x2': Grid2x2,
   home: House,
   'layout-dashboard': LayoutDashboard,
+  'log-in': LogIn,
   'log-out': LogOut,
   mail: Mail,
   monitor: Monitor,