Fix two sensor graph footer issues

[The0x539]

Before:
After:

• If a (non-open) footer isn't full width, there is a gap between the .info and .footer elements that gets filled with the background-color (which is also what determines the inner-color color, so we can't just set it to black).
  • Fixed by using flexbox to make the footer element (but not the graph inside) grow to fill the gap.
• The graph draws on top of the inner corner, causing subtle discoloration.
  • Fixed by using overflow: hidden to clip the graph to the black region.

I originally discovered the first issue by creating a horizontal stack of two sensors at the end of a vertical stack. I noticed the second issue while investigating the first.

I may have put these rules in a more specific rule than would be ideal, but as far as I know this is the only situation with this issue. Graphs as headers have completely different brokenness that I'm less interested in personally investigating.

Test case:

type: sensor
graph: line
entity: sensor.husky_battery_level
card_mod:
  class: middle-right
layout_options:
  grid_columns: 3
  grid_rows: 3

[th3jesta]

Excellent! I can probably employ this method in other card types, as well. It's way less janky than some of the fixes I've implemented for similar issues.