Theme pairs with dark mode toggle, gear active state, PALETTE_DEFAULT_DARK (#127)

- Collapse the 10 theme cards into 5 pairs: light swatches top row, dark bottom row
- Dark mode toggle in settings and topbar (sun/moon inline SVG)
- Toggling dark mode switches to the other variant of the selected pair
- Persist pair id in localStorage palette-theme, dark flag in palette-dark
- Head script resolves base+dark to variant; URL ?theme= accepts both ids and wins
- Settings gear highlights like nav tabs on /settings
- PALETTE_DEFAULT_DARK env var sets server default dark state (default on)
- Server-side resolution helpers + tests

internal/web/web_test.go

@@ -0,0 +1,48 @@
+package web
+
+import "testing"
+
+func TestResolvePreset(t *testing.T) {
+	cases := []struct {
+		pair string
+		dark bool
+		want string
+	}{
+		{"", false, "midnight-light"},
+		{"", true, "midnight"},
+		{"midnight", false, "midnight-light"},
+		{"midnight", true, "midnight"},
+		{"smooth", false, "smooth"},
+		{"smooth", true, "smooth-dark"},
+		{"pastel-lavender", true, "pastel-lavender-dark"},
+		{"pastel-peach", false, "pastel-peach"},
+		{"pastel-cloud", true, "pastel-cloud-dark"},
+		{"bogus", true, "midnight"},
+		{"bogus", false, "midnight-light"},
+		// explicit variant ids also resolve (URL ?theme= may pass them)
+		{"smooth-dark", true, "midnight"},
+	}
+	for _, c := range cases {
+		if got := ResolvePreset(c.pair, c.dark); got != c.want {
+			t.Errorf("ResolvePreset(%q, %v) = %q, want %q", c.pair, c.dark, got, c.want)
+		}
+	}
+}
+
+func TestSetDefaultDark(t *testing.T) {
+	for _, off := range []string{"false", "0", "off", "OFF", " false "} {
+		DefaultDark = true
+		SetDefaultDark(off)
+		if DefaultDark {
+			t.Errorf("SetDefaultDark(%q): want dark off", off)
+		}
+	}
+	for _, on := range []string{"true", "1", "on", "anything"} {
+		DefaultDark = false
+		SetDefaultDark(on)
+		if !DefaultDark {
+			t.Errorf("SetDefaultDark(%q): want dark on", on)
+		}
+	}
+	DefaultDark = true
+}