fix(sensors): keep text contrast against the real background

[perlowja]

The sensors chip colors each metric from theme tokens without looking at what is behind it, and the popover dims values with dim-label opacity. On a translucent panel over a bright wallpaper, or a light theme over a dark surface, the text is close to invisible. Measured with the WCAG formula: red on sky blue 1.7:1, green on sky blue 1.4:1, green on a light panel 1.8:1, dim value on a light popover 3.8:1.

Change

• sensors/contrast.vala: WCAG luminance and contrast math, lightness shifting that keeps the theme hue, a muted tone, a median helper.
• The chip and popover sample the pixels around themselves and derive neutral, muted, ok, warning, critical and accent colors at 7:1 where possible, 4.5:1 minimum. A translucent surface gets its own backing.
• Warning and critical also differ by glyph and weight, not hue alone.
• Re-evaluated on theme, dark-preference and color-scheme changes, panel redraw (150 ms debounce) and popover open. No polling.
• Only GTK4 and libsingularity. One concern.

Before / after (dark, light):

before	after

Verification

• Commit 8a495f6 on current main (d255d47) with libsingularity b454931, shell 3b72f26 and desktop master 5c75e56, Debian forky arm64 container: meson setup && ninja 784/784 targets, meson test 25/25 pass, including sensors-contrast (7 cases over 10 backgrounds; neutral, muted and semantic colors asserted at 4.5:1 or better, white and black at 7:1).
• Two x86_64 laptops (AMD Navi14, GTK 4.22, labwc): dark and light theme, live theme switch without restart. Also run on an aarch64 board (dark and light chip, dark popover).

Not verified

• Warning and critical states were only exercised by the unit test; the test machines never reached those readings.
• A translucent panel cannot see the wallpaper; the backing follows the theme background polarity.
• This touches the same file as feat(sensors): DPI-aware column density, bordered sections #2; I will rebase whichever lands second.

AI assistance: disclosed