fix(cliamp hook): readable accent + correct fg/bright_fg brightness

[HANCORE-linux]

Problem

The cliamp theme-set hook (theme-set.d/50-cliamp.sh) writes
~/.config/cliamp/themes/omarchy.toml with a verbatim mapping:

accent    = normal_blue    # color4
bright_fg = bright_white   # color15
fg        = normal_white   # color7

This breaks colours for some Omarchy themes, with the only workaround being
rm -rf ~/.config/cliamp/themes/. Two issues, measured on a matte/monochrome
theme (background #121212):

1. accent = color4 is unreadable. cliamp uses accent for the title, the
   seek bar and the key-hint background. On monochrome palettes color4 is a
   dark colour (e.g. #605d5b, contrast 2.87:1 vs bg, below WCAG 3:1) →
   those elements become invisible.
2. bright_fg/fg aren't ordered by brightness. cliamp uses bright_fg
   for normal text and fg for dimmed text, so bright_fg must be the brighter
   of the two. Some palettes define color15 darker than color7
   (#a4a4a4 vs #dbd9d3), so dimmed text ends up brighter than normal
   text → "wrong / too bright" colours.

(rm only "helps" because cliamp then can't find the theme, SetTheme returns
false, and cliamp falls back to its ANSI defaults.)

Fix

• Pick bright_fg/fg from color7/color15 by luminance, so bright_fg
  is always the brighter one.
• Keep accent = normal_blue when it has enough contrast against the
  background; otherwise fall back to the first readable, preferably coloured
  candidate (bright_blue, normal_cyan, normal_yellow, normal_green,
  normal_red), and only to the foreground colour as a last resort.

Pure integer luminance, no new dependencies. Verified on the affected theme:
accent #605d5b → #b7a798 (readable), bright_fg #a4a4a4 → #dbd9d3,
fg #dbd9d3 → #a4a4a4 (correct order).

Context: this was originally (mis)filed as a cliamp validation issue
(bjarneo/cliamp#260) — the user themes are complete and valid; the real fix is
here in the hook's colour mapping.