Fix low temperature text visibility on dark terminal backgrounds

[Copilot]

Motivation and Context

Cold temperature values (< -3°C) were rendered in deep-blue ANSI 256 colors (21 = #0000ff, 27 = #005fff) that are nearly invisible against dark terminal backgrounds, as shown above.

Description

• frontends/colors.go (new) — darkBackground() reads COLORFGBG (set by rxvt, xterm, etc.; format "fg;bg" or "fg;default;bg") to detect terminal background. Returns true (dark) when unset, since dark terminals are the dominant use case. Dark = color index 0–6 or 8; light = 7 or 9–15.

• frontends/ascii-art-table.go and frontends/emoji.go — colmap hoisted outside the color closure so it can be patched before use. On dark backgrounds, the 5 cold-temperature color entries are replaced:

  Temp	Before (dark bg)	After (dark bg)
  < −15°C	21 #0000ff ☠️	33 #0087ff
  −15 to −12	27 #005fff ☠️	39 #00afff
  −12 to −9	33	45 #00d7ff
  −9 to −6	39	51 #00ffff
  −6 to −3	45	51 #00ffff

  Light backgrounds keep the original darker blues. Warm-end colors are untouched.

Steps for Testing

1. On a dark-background terminal (no COLORFGBG set, or COLORFGBG=15;0): run wego with a location that has sub-zero temperatures — cold temp values should now render in clearly visible cyan-blue.
2. On a light-background terminal (COLORFGBG=0;15): cold temps should use the original deep-blue palette.
3. Manually set COLORFGBG=0;15 / COLORFGBG=15;0 to toggle behavior without changing terminal theme.

Screenshots

Before (dark background — cold temperatures barely visible):