refactor: use __fish_palette in the config-settings TUI

27 duplicated declarations replaced by 5 calls. Output strings untouched.

Four of these five files have no --help path, so the byte-identity harness
cannot reach them. Covered instead by driving the real TUI under a pty on
both the baseline and converted trees with an identical paced keystroke
feed (down x3, Tab, Enter, Esc, Tab, down, q):

  9 redraws, 15068 bytes, byte-identical on both sides
  c_sel (ESC[35;1m) x9, c_hi (ESC[37;1m) x8, c_head (ESC[36;1m) x9
  all four draw helpers reached -- draw, pagetab, draw_subcat
  ("cascade default", "(category)"), draw_value ("Allow prev")

functions/config-settings.fish

@@ -69,12 +69,7 @@
 # EXAMPLE
 #   config-settings
 function config-settings --description 'Interactive TUI for managing fish config settings'
-    set -l c_head  (set_color --bold cyan)
-    set -l c_cmd   (set_color --bold)
-    set -l c_flag  (set_color yellow)
-    set -l c_dim   (set_color brblack)
-    set -l c_err   (set_color red)
-    set -l c_reset (set_color normal)
+    __fish_palette
 
     # ── Argument parsing ──────────────────────────────────
     for arg in $argv