fix(flow-mobile): Increase specificity of fill-modifying rule

[gadenbuie]

Counterpart to rstudio/htmltools#425. Technically that PR fixes the same issue (and contains further details), but we might not want a hard depdendency on a new version of htmltools.

No news entry because this fixes a not-yet-released bug.

Example app

```r library(shiny) pkgload::load_all() ui <- page_fillable( layout_columns( col_widths = c(12, 6, 6), row_heights = breakpoints( xs = c(7, 1.5, 1.5), sm = c(70, 30) ), card("Chart 1"), navset_card_tab( nav_panel("Chart 2"), nav_panel("Chart 3") ), card("Chart 4") ) ) server <- function(input, output, session) { } shinyApp(ui, server) ```

[gadenbuie]

I wouldn't be entirely opposed to just requiring the latest htmltools, but I don't mind either way

We already require the latest htmltools release in bslib, so I think bumping the version requirement is the best way forward here. Will submit a PR shortly