Add Crimson theme: red on OLED black

Pure #000 background (OLED pixels off), near-black panels with dark red
borders, and a red accent (#e0263a) that passes 4.5:1 as text on black and
behind white button labels. Badges use a brighter red to stay legible on
their tinted background.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/themes.go

@@ -18,6 +18,7 @@ var Themes = []Theme{
 	{"gruvbox", "Gruvbox", true},
 	{"forest", "Forest", true},
 	{"synthwave", "Synthwave", true},
+	{"crimson", "Crimson", true},
 	{"daylight", "Daylight", false},
 	{"paper", "Paper", false},
 	{"latte", "Latte", false},