Fix unreadable event text when a calendar's color is white/very light

The month view rendered each event's text (and its little leading dot)
in the calendar's own color, on a very lightly tinted (~13% opacity)
background of that same color. For a light color like white or pale
yellow, the text ended up effectively invisible against its own
near-white background.

internal/web/templates/calendar.templ: add eventTextColor(hex), which
darkens colors above a perceived-luminance threshold (keeping the hue,
e.g. white -> a mid gray, pale yellow -> olive) while leaving already-
legible colors untouched; used for both the event text and its leading
dot. Also add a subtle ring to the event dot and to the calendar legend's
color swatch so a white/near-white swatch stays visible against the
page's white background, independent of the text-color fix.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/web/templates/calendar.templ

@@ -1,6 +1,8 @@
 package templates
 
 import "fmt"
+import "strconv"
+import "strings"
 
 // CalendarSummary is one calendar (own or shared) shown in the combined
 // month view's legend.
@@ -119,7 +121,7 @@ templ MonthView(username string, data MonthViewData) {
 
 templ calendarLegendRow(c CalendarSummary) {
 	<div class="px-4 py-3 flex items-center gap-3 text-sm flex-wrap">
-		<span class="w-3 h-3 rounded-full shrink-0" style={ "background-color: " + colorOrDefault(c.Color) }></span>
+		<span class="w-3 h-3 rounded-full shrink-0 ring-1 ring-inset ring-black/10" style={ "background-color: " + colorOrDefault(c.Color) }></span>
 		<span class="font-medium">{ c.Name }</span>
 		if c.Shared {
 			<span class="text-xs text-gray-500">shared by { c.Owner }</span>
@@ -155,10 +157,10 @@ templ monthDayCell(day MonthDay) {
 			<a
 				href={ templ.URL(eventLinkURL(ev)) }
 				class="block truncate rounded px-1.5 py-0.5 text-xs hover:opacity-80"
-				style={ "background-color: " + colorOrDefault(ev.Color) + "22; color: " + colorOrDefault(ev.Color) }
+				style={ "background-color: " + colorOrDefault(ev.Color) + "22; color: " + eventTextColor(ev.Color) }
 				title={ ev.Summary }
 			>
-				<span class="inline-block w-1.5 h-1.5 rounded-full mr-1" style={ "background-color: " + colorOrDefault(ev.Color) }></span>
+				<span class="inline-block w-1.5 h-1.5 rounded-full mr-1 ring-1 ring-inset ring-black/10" style={ "background-color: " + eventTextColor(ev.Color) }></span>
 				if !ev.AllDay && ev.TimeText != "" {
 					<span class="font-medium">{ ev.TimeText }</span>
 				}
@@ -175,6 +177,55 @@ func eventLinkURL(ev EventSummary) string {
 	return "/web/calendar/" + ev.CalRef + "/" + ev.ID + "/edit"
 }
 
+// eventTextColor returns a color derived from hex, darkened if needed so
+// it stays legible as text on monthDayCell's very light (~13% opacity)
+// tinted event background. Without this, a light calendar color like
+// white or pale yellow renders its own event text as invisible or
+// near-invisible, since the same color is used for both the tint and the
+// text. Colors that are already dark enough are returned unchanged.
+func eventTextColor(hex string) string {
+	c := colorOrDefault(hex)
+	r, g, b, ok := parseHexColor(c)
+	if !ok {
+		return c
+	}
+	// Perceived luminance (ITU-R BT.601).
+	luminance := 0.299*float64(r) + 0.587*float64(g) + 0.114*float64(b)
+	const maxReadableLuminance = 170
+	if luminance <= maxReadableLuminance {
+		return c
+	}
+	// Scale each channel down so the color darkens towards black while
+	// keeping its hue, ending up comfortably below the threshold.
+	scale := maxReadableLuminance / luminance * 0.75
+	return fmt.Sprintf("#%02x%02x%02x", clampByte(float64(r)*scale), clampByte(float64(g)*scale), clampByte(float64(b)*scale))
+}
+
+// parseHexColor parses a "#rrggbb" string into its red/green/blue
+// components. ok is false for anything else (including 3-digit shorthand
+// hex, which colorOrDefault/hexColorRe never produce).
+func parseHexColor(s string) (r, g, b int, ok bool) {
+	s = strings.TrimPrefix(s, "#")
+	if len(s) != 6 {
+		return 0, 0, 0, false
+	}
+	v, err := strconv.ParseInt(s, 16, 32)
+	if err != nil {
+		return 0, 0, 0, false
+	}
+	return int(v >> 16 & 0xff), int(v >> 8 & 0xff), int(v & 0xff), true
+}
+
+func clampByte(v float64) int {
+	if v < 0 {
+		return 0
+	}
+	if v > 255 {
+		return 255
+	}
+	return int(v)
+}
+
 
 
 templ EventForm(username string, data EventFormData, errMsg string) {