feat(web): board and calendar views, saved filters and label pages

The board is one column per section with native drag-and-drop, which
works with a trackpad, a mouse and a screen reader without any of them
being reimplemented. Dropping is optimistic: a card that snaps back for
200ms reads as a failed drop.

Weeks in the calendar start on Monday. That is ISO 8601 and what every
Danish calendar does; a Sunday-first grid makes a Dane misread the month
at a glance. On a phone the chips give way to coloured bars — at that
width the month's shape is the useful part, not the titles.

Label pages run the filter language rather than a bespoke endpoint, so a
label page and a saved filter can never disagree about what a label
selects.

fix(recurrence): "hver mandag", not "hver uge mandag"

A weekly rule that names its days does not also need the word for the
interval. That is how the phrase is said, and it is what the parser
accepts back — the round trip through Describe now holds for weekday
rules too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/recurrence/recurrence_test.go

@@ -270,8 +270,9 @@ func TestDescribe(t *testing.T) {
 		"FREQ=DAILY;INTERVAL=3":            "hver 3. dag",
 		"FREQ=WEEKLY":                      "hver uge",
 		"FREQ=WEEKLY;INTERVAL=2":           "hver anden uge",
-		"FREQ=WEEKLY;BYDAY=MO":             "hver uge mandag",
-		"FREQ=WEEKLY;BYDAY=MO,TH":          "hver uge mandag og torsdag",
+		"FREQ=WEEKLY;BYDAY=MO":             "hver mandag",
+		"FREQ=WEEKLY;BYDAY=MO,TH":          "hver mandag og torsdag",
+		"FREQ=WEEKLY;BYDAY=FR;INTERVAL=2":  "hver anden uge fredag",
 		"FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR": "hverdage",
 		"FREQ=WEEKLY;BYDAY=SA,SU":          "hver weekend",
 		"FREQ=MONTHLY":                     "hver måned",
@@ -293,6 +294,7 @@ func TestDescribeRoundTrip(t *testing.T) {
 	phrases := []string{
 		"hver dag", "hver anden dag", "hver uge", "hver anden uge",
 		"hverdage", "hver weekend", "hver måned", "hvert år",
+		"hver mandag", "hver mandag og torsdag",
 	}
 	for _, phrase := range phrases {
 		t.Run(phrase, func(t *testing.T) {