Fix schedule navigation

Previously, the Session depended on the page to return a .navigate state when
a secondary navigation should take place. There were a lot of places where the
page returned .done rather than .navigate.

To simplify the code, .navigate is no longer a WaitResult. The page now just has
to return .done, and the Session will check if there's a queued sub-navigation.

This will fix cases where sub-navigation is triggered by a script being executed
as the last step of page loading.

src/Server.zig

@@ -205,7 +205,6 @@ fn readLoop(self: *Server, socket: posix.socket_t, timeout_ms: u32) !void {
                 }
                 ms_remaining -= @intCast(elapsed);
             },
-            .navigate => unreachable, // must have been handled by the session
         }
     }
 }