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.

3 files changed, 19 insertions, 16 deletions

diff --git a/internal/service/media.go b/internal/service/media.go
index f950db3..6f4b2db 100644
--- a/internal/service/media.go
+++ b/internal/service/media.go
@@ -35,8 +35,11 @@ func NewMediaService(store repository.MediaServiceStore, clk clock.Clock, mediaR
 
 // Sentinel errors returned by the media service layer.
 var (
-	ErrNotFound  = errors.New("not found")
-	ErrForbidden = errors.New("access denied")
+	ErrNotFound      = errors.New("not found")
+	ErrForbidden     = errors.New("access denied")
+	ErrShareNotFound = errors.New("share not found")
+	ErrShareExpired  = errors.New("share expired")
+	ErrMediaNotFound = errors.New("media not found")
 )
 
 func (s *mediaService) ListSets(ctx context.Context, userID int64) ([]model.Set, error) {
@@ -510,16 +513,16 @@ func (s *mediaService) ValidateShareToken(ctx context.Context, token string) (*m
 		return nil, fmt.Errorf("get share: %w", err)
 	}
 	if share == nil {
-		return nil, nil
+		return nil, ErrShareNotFound
 	}
 
 	now := s.clock.Now()
 	if now.After(share.ExpiresAt) {
-		return nil, nil
+		return nil, ErrShareExpired
 	}
 
 	if share.MaxUses != nil && share.UsedCount >= *share.MaxUses {
-		return nil, nil
+		return nil, ErrShareExpired
 	}
 
 	return share, nil
@@ -530,16 +533,13 @@ func (s *mediaService) StreamSharedMedia(ctx context.Context, token string) (*Fi
 	if err != nil {
 		return nil, err
 	}
-	if share == nil {
-		return nil, errors.New("invalid or expired share")
-	}
 
 	media, err := s.store.GetMediaByID(ctx, share.MediaID)
 	if err != nil {
 		return nil, fmt.Errorf("get media: %w", err)
 	}
 	if media == nil {
-		return nil, errors.New("media not found")
+		return nil, ErrMediaNotFound
 	}
 
 	_ = s.store.UseShare(ctx, token)
diff --git a/internal/service/media_test.go b/internal/service/media_test.go
index 1ab7dfb..33912b4 100644
--- a/internal/service/media_test.go
+++ b/internal/service/media_test.go
@@ -905,14 +905,17 @@ func TestMediaService_ValidateShareToken(t *testing.T) {
 			}
 			svc := NewMediaService(store, newMockClock(), "/tmp/media")
 			res, err := svc.ValidateShareToken(ctx, "abc")
-			if err != nil {
-				t.Fatalf("unexpected error: %v", err)
-			}
 			if tt.wantValid {
+				if err != nil {
+					t.Fatalf("unexpected error: %v", err)
+				}
 				if res == nil {
 					t.Fatal("expected valid share")
 				}
 			} else {
+				if err == nil {
+					t.Fatal("expected error for invalid share")
+				}
 				if res != nil {
 					t.Fatal("expected nil share")
 				}
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)
 		}
 	})