ja: implement public share landing page and improve error mapping

- Add web/share.html with a minimal HTML5 video player, theme variables,
  centered layout, play overlay, and Back to Home link.
- Update handleSharePage to return HTML for browser Accept headers
  (text/html or empty) and JSON for application/json. Return 410 Gone
  for expired shares.
- Update handleShareStream to map service errors to distinct HTTP codes:
  ErrShareExpired -> 410, ErrShareNotFound/ErrMediaNotFound -> 404.
- Improve ValidateShareToken and StreamSharedMedia to return sentinel
  errors (ErrShareNotFound, ErrShareExpired, ErrMediaNotFound) instead
  of generic string errors.
- Update and add tests for share page HTML/JSON negotiation and share
  stream 404/410 responses.

1 files changed, 4 insertions, 4 deletions

diff --git a/internal/service/no_rows_test.go b/internal/service/no_rows_test.go
index 3606b63..9200668 100644
--- a/internal/service/no_rows_test.go
+++ b/internal/service/no_rows_test.go
@@ -135,8 +135,8 @@ func TestService_NoRows_ReturnsNil(t *testing.T) {
 		}
 		svc := NewMediaService(store, newMockClock(), "/tmp/media")
 		sh, err := svc.ValidateShareToken(ctx, "nope")
-		if err != nil {
-			t.Fatalf("expected no error, got %v", err)
+		if !errors.Is(err, ErrShareNotFound) {
+			t.Fatalf("expected ErrShareNotFound, got %v", err)
 		}
 		if sh != nil {
 			t.Fatalf("expected nil share, got %+v", sh)
@@ -153,8 +153,8 @@ func TestService_NoRows_ReturnsNil(t *testing.T) {
 		}
 		svc := NewMediaService(store, newMockClock(), "/tmp/media")
 		_, err := svc.StreamSharedMedia(ctx, "nope")
-		if err == nil {
-			t.Fatal("expected error for missing share")
+		if !errors.Is(err, ErrShareNotFound) {
+			t.Fatalf("expected ErrShareNotFound, got %v", err)
 		}
 	})