feat(sponsors): drop the sidebar slot and simplify the Sponsors page

The sidebar no longer carries a sponsor card. The backend stops serving
the "sidebar" slot, so a sponsors.json entry that still lists it keeps
only its other slots and one listing nothing else is dropped. Its
rotation, three-sponsor cap and collapsed-rail logo rendering go with it.

The Sponsors page now lists only real sponsors: the "Your brand here"
card shows only as the empty state instead of sitting in the grid and
repeating the header's Become a sponsor button, and the Available
placements section and its slot-occupancy helper are removed. Cards are
larger, two per row on wide screens. Unused i18n keys are dropped from
every locale.

internal/web/service/panel/sponsor.go

@@ -45,7 +45,6 @@ const (
 	sponsorsErrTTL   = 10 * time.Minute
 	maxSponsorsBytes = 256 << 10
 	maxLogoBytes     = 256 << 10
-	maxSidebarSlots  = 3
 	localSponsorsDir = "../sponsors/3X"
 	sponsorLogoPath  = "/sponsors/logo/"
 )
@@ -67,7 +66,7 @@ var (
 
 	// The panel proxies logos from sponsorLogoBase: CSP stays 'self' and no third party sees admin IPs.
 	sponsorLogoRe = regexp.MustCompile(`^[A-Za-z0-9_-][A-Za-z0-9._-]*\.(png|webp|jpg)$`)
-	sponsorSlots  = map[string]bool{"dashboard": true, "sidebar": true, "page": true, "login": true}
+	sponsorSlots  = map[string]bool{"dashboard": true, "page": true, "login": true}
 
 	sponsorsMu      sync.Mutex
 	sponsorsRaw     *SponsorList
@@ -217,7 +216,6 @@ func activeSponsors(raw *SponsorList, now time.Time) *SponsorList {
 	if strings.HasPrefix(raw.Contact, "https://") {
 		out.Contact = raw.Contact
 	}
-	sidebarTaken := 0
 	for _, sp := range raw.Sponsors {
 		// A missing enable counts as on, so a forgotten field never hides a paid slot.
 		disabled := sp.Enable != nil && !*sp.Enable
@@ -233,18 +231,13 @@ func activeSponsors(raw *SponsorList, now time.Time) *SponsorList {
 		}
 		slots := make([]string, 0, len(sp.Slots))
 		for _, slot := range sp.Slots {
-			// The sidebar rotates, so capping it keeps each paid card on screen long enough.
-			if !sponsorSlots[slot] || (slot == "sidebar" && sidebarTaken >= maxSidebarSlots) {
-				continue
+			if sponsorSlots[slot] {
+				slots = append(slots, slot)
 			}
-			slots = append(slots, slot)
 		}
 		if len(slots) == 0 {
 			continue
 		}
-		if slices.Contains(slots, "sidebar") {
-			sidebarTaken++
-		}
 		sp.Slots = slots
 		out.Sponsors = append(out.Sponsors, sp)
 	}

internal/web/service/panel/sponsor_test.go

@@ -6,7 +6,6 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
-	"slices"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -22,7 +21,7 @@ func validSponsor() Sponsor {
 	return Sponsor{
 		ID:    "acme",
 		Name:  "Acme",
-		Slots: []string{"dashboard", "sidebar"},
+		Slots: []string{"dashboard"},
 		Until: sponsorTestNow.Add(24 * time.Hour),
 		Logo:  "acme.png",
 		Link:  "https://acme.example/",
@@ -59,34 +58,6 @@ func TestActiveSponsorsFilters(t *testing.T) {
 	}
 }
 
-func TestActiveSponsorsCapsSidebarAtThree(t *testing.T) {
-	raw := &SponsorList{}
-	for _, id := range []string{"expired", "a", "b", "c", "d", "e"} {
-		sp := validSponsor()
-		sp.ID = id
-		sp.Slots = []string{"sidebar", "page"}
-		if id == "expired" {
-			sp.Until = sponsorTestNow
-		}
-		if id == "e" {
-			sp.Slots = []string{"sidebar"}
-		}
-		raw.Sponsors = append(raw.Sponsors, sp)
-	}
-	got := activeSponsors(raw, sponsorTestNow)
-	want := map[string][]string{
-		"a": {"sidebar", "page"}, "b": {"sidebar", "page"}, "c": {"sidebar", "page"}, "d": {"page"},
-	}
-	if len(got.Sponsors) != len(want) {
-		t.Fatalf("got %d sponsors, want %d (e has only sidebar and must drop)", len(got.Sponsors), len(want))
-	}
-	for _, sp := range got.Sponsors {
-		if !slices.Equal(sp.Slots, want[sp.ID]) {
-			t.Errorf("%s slots = %v, want %v", sp.ID, sp.Slots, want[sp.ID])
-		}
-	}
-}
-
 func TestActiveSponsorsLogoName(t *testing.T) {
 	cases := []struct{ logo, want string }{
 		{"acme.png", "/sponsors/logo/acme.png"},
@@ -114,7 +85,7 @@ func TestActiveSponsorsLogoName(t *testing.T) {
 
 func TestActiveSponsorsResolvesLogoAndSlots(t *testing.T) {
 	sp := validSponsor()
-	sp.Slots = []string{"subpage", "login"}
+	sp.Slots = []string{"subpage", "sidebar", "login"}
 	got := activeSponsors(&SponsorList{Contact: "javascript:x", Sponsors: []Sponsor{sp}}, sponsorTestNow)
 	if len(got.Sponsors) != 1 {
 		t.Fatalf("got %d sponsors, want 1", len(got.Sponsors))