Add calendar color support (DAVx5 calendar-color)

Calendars can now have a color (hex, e.g. #3b82f6) that DAVx5 and other
CalDAV clients pick up via the Apple/dav4jvm calendar-color property.

- db: add calendars.color column with migration for existing DBs;
  CreateCalendarWithColor, SetCalendarColor, GetCalendarColor;
  ListCalendars now returns []Calendar{Name, Color} instead of []string
- caldav: since go-webdav's caldav.Backend interface has no extension
  point for vendor properties, wrap the handler with a response-rewriting
  middleware that injects <calendar-color xmlns="http://apple.com/ns/ical/">
  into PROPFIND responses for calendars that have a color set
- web: color picker on the "New calendar" form and an inline color swatch/
  picker on each calendar card (calendars only, not address books)
- nidusctl: `calendar create --color` flag and a new `calendar color`
  subcommand; `calendar list` now also prints the color if set

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

internal/web/templates/dashboard.templ

@@ -13,6 +13,7 @@ type ShareRow struct {
 type ResourceCard struct {
 	Kind   string // "calendar" or "addressbook"
 	Name   string
+	Color  string // hex color like "#3b82f6"; only used for calendars
 	Shares []ShareRow
 }
 
@@ -43,6 +44,11 @@ templ Dashboard(username string, resources []ResourceCard, sharedWithMe []Shared
 						placeholder="e.g. work"
 						class="w-full rounded-md border-gray-300 border px-2 py-1.5 text-sm"/>
 				</div>
+				<div>
+					<label class="block text-xs text-gray-500 mb-1">Color</label>
+					<input name="color" type="color" value="#3b82f6"
+						class="w-12 h-9 rounded-md border-gray-300 border p-0.5"/>
+				</div>
 				<button type="submit"
 					class="w-full sm:w-auto bg-indigo-600 text-white rounded-md px-3 py-1.5 text-sm font-medium hover:bg-indigo-700">
 					Add
@@ -106,7 +112,24 @@ templ ResourceList(resources []ResourceCard) {
 templ ResourceCardView(r ResourceCard) {
 	<div id={ "resource-" + r.Kind + "-" + r.Name } class="bg-white rounded-lg border border-gray-200 p-5">
 		<div class="flex items-center justify-between mb-3">
-			<h2 class="font-medium">
+			<h2 class="font-medium flex items-center gap-2">
+				if r.Kind == "calendar" {
+					<form
+						hx-post="/web/resources/calendar/color"
+						hx-target={ "#resource-" + r.Kind + "-" + r.Name }
+						hx-swap="outerHTML"
+						hx-trigger="change"
+					>
+						<input type="hidden" name="name" value={ r.Name }/>
+						<input
+							name="color"
+							type="color"
+							value={ colorOrDefault(r.Color) }
+							title="Calendar color"
+							class="w-6 h-6 rounded border border-gray-300 p-0 align-middle"
+						/>
+					</form>
+				}
 				{ r.Name }
 				<span class="text-xs uppercase tracking-wide text-gray-400 ml-2">{ r.Kind }</span>
 			</h2>
@@ -195,3 +218,10 @@ func resourceVals(name string) string {
 	return `{"name": "` + name + `"}`
 }
 
+func colorOrDefault(c string) string {
+	if c == "" {
+		return "#3b82f6"
+	}
+	return c
+}
+