Fix invisible select chevron in all presets (#136)

Two compounding causes:
- CSP img-src fell back to default-src (self), blocking the data: URI
  chevron image on all served pages, so no select ever painted it.
- .editor-head select used the background shorthand, wiping the
  background-image set by the later select rule.

Chevron stroke is now a per-preset --chevron token (light stroke for
dark presets, dark for light presets) instead of hardcoded #888.
CSP gains img-src 'self' data: so the data-URI icons can load.

internal/web/securityheaders_test.go

@@ -16,7 +16,7 @@ func TestSecurityHeaders(t *testing.T) {
 	h := SecurityHeaders(pages)
 	rec := httptest.NewRecorder()
 	h.ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
-	wantCSP := "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; frame-ancestors 'none'"
+	wantCSP := "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'none'"
 	if got := rec.Header().Get("Content-Security-Policy"); got != wantCSP {
 		t.Errorf("CSP = %q, want %q", got, wantCSP)
 	}