fix(wallpaper): make sparkline graph backgrounds respect dashboard opacity

Sparkline::draw (CPU/GPU load history) and net.rs's own draw_dual (network
up/down) both painted a hardcoded, always-opaque background rect independent
of the panel opacity plumbing added for #131 — visible as solid black boxes
behind the mini-graphs at reduced opacity in Desktop Wallpaper mode. Thread
opacity through both and premultiply the background fill plus the line/
gradient colors with the existing premul_color helper, matching how the rest
of each panel already blends.

Closes #168