Make unsaved Nearby networks clickable, add password-entry dialog

Rows for scanned networks not yet in breadcrumbs' saved config were
both dimmed and disabled (set_sensitive(false)), so there was no way
to add them from the popover. They now stay visually dimmed via the
existing wifi-popover-row-unsaved style but remain clickable, opening
a small password dialog that saves + joins the network via
`breadcrumbs add` + `breadcrumbs join`.

src/theme.rs

@@ -50,6 +50,7 @@ fn load_css() -> String {
          .wifi-popover-row-active {{ color: {accent}; }}\
          .wifi-popover-row-unsaved {{ opacity: 0.4; }}\
          .wifi-popover-loading {{ opacity: 0.5; padding: 8px; }}\
+         window.wifi-add-dialog {{ background-color: {bg_rgba}; color: {on_bg}; min-width: 240px; }}\
          .media-widget {{ border-radius: 4px; padding: 0 6px; cursor: pointer; }}\
          .media-widget:hover {{ background: alpha({on_bg}, 0.10); }}\
          .media-indicator {{ font-size: 11px; opacity: 0.7; margin-right: 2px; }}\