feat: rebuild navbar with NB structure — Sis brand, nav links with nb-navbar-link class inside nb-navbar-item wrappers, user initials avatar

- UserSession now carries usDisplayName for extracting initials (current: JB)
- Nav items: Today (was Dashboard), Chores, Activity, Household
- Removed Logout link; added black user avatar circle with initials
- routeLink generates proper <li class='nb-navbar-item'><a class='nb-navbar-link'> structure
- All pages wrapped with pageLayout to render shared navbar shell
- Login/Signup save display name into session on authentication

src/Sis/Page/Activity.hs

@@ -101,7 +101,7 @@ page = do
         Just us -> do
             hhs <- getUserHouseholds (UserId (usUserId us))
             case hhs of
-                [] -> pure $ hyper ActivityPage $ el "No households"
+                [] -> pure $ hyper ActivityPage $ pageLayout us $ el "No households"
                 (h : _) -> do
                     log <- getActivityLog (unHouseholdId (householdId h)) 1 20
-                    pure $ hyper ActivityPage $ activityView log
+                    pure $ hyper ActivityPage $ pageLayout us $ activityView log