Fix layouting issue on welcome screen for narrow window, triggering d…

…ebug assertion (#5650)

### What

* Fixes #5649

~This is a speculative fix, didn't have a repro.~ From the callstack we
know that a certain `ui.set_max_width(column_width - 8.0);` might have
gotten a too small width. Looking at how `column_width` is produced this
may very well happen if for some reason there's very little
`available_width`.

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crates/re_viewer/src/ui/welcome_screen/welcome_section.rs

@@ -192,6 +192,7 @@ fn onboarding_content_ui(ui: &mut Ui, command_sender: &CommandSender, accepts_co
     let column_width = ((ui.available_width() + grid_spacing.x) / column_count as f32
         - grid_spacing.x)
         .floor()
+        .at_least(MIN_COLUMN_WIDTH)
         .at_most(MAX_COLUMN_WIDTH);
 
     ui.horizontal(|ui| {