fix(shares): honor path component of ShareURL config

PublicURL() copied only the scheme and host from conf.Server.ShareURL,
silently dropping any path component. This broke OpenGraph image URLs
(and other share links) when ShareURL was configured with a path prefix
like https://example.com/navi — generated URLs pointed to /share/img/...
at the root instead of /navi/share/img/...

Now the ShareURL path is prepended to the resource path, with trailing
slashes trimmed. When ShareURL has no path, behavior is unchanged.

core/publicurl/publicurl.go

@@ -45,6 +45,9 @@ func PublicURL(req *http.Request, u string, params url.Values) string {
 	}
 	buildUrl.Scheme = shareUrl.Scheme
 	buildUrl.Host = shareUrl.Host
+	if basePath := strings.TrimRight(shareUrl.Path, "/"); basePath != "" {
+		buildUrl.Path = path.Join(basePath, buildUrl.Path)
+	}
 	if len(params) > 0 {
 		buildUrl.RawQuery = params.Encode()
 	}

core/publicurl/publicurl_test.go

@@ -56,6 +56,31 @@ var _ = Describe("Public URL Utilities", func() {
 			})
 		})
 
+		When("ShareURL includes a path", func() {
+			BeforeEach(func() {
+				conf.Server.ShareURL = "https://example.com/navi"
+			})
+
+			It("prepends the ShareURL path to the resource", func() {
+				r, _ := http.NewRequest("GET", "http://localhost/test", nil)
+				result := publicurl.PublicURL(r, "/share/img/hash", nil)
+				Expect(result).To(Equal("https://example.com/navi/share/img/hash"))
+			})
+
+			It("prepends the ShareURL path and includes query parameters", func() {
+				r, _ := http.NewRequest("GET", "http://localhost/test", nil)
+				params := url.Values{"size": []string{"600"}}
+				result := publicurl.PublicURL(r, "/share/img/hash", params)
+				Expect(result).To(Equal("https://example.com/navi/share/img/hash?size=600"))
+			})
+
+			It("handles trailing slash in ShareURL path", func() {
+				conf.Server.ShareURL = "https://example.com/navi/"
+				result := publicurl.PublicURL(nil, "/share/img/hash", nil)
+				Expect(result).To(Equal("https://example.com/navi/share/img/hash"))
+			})
+		})
+
 		When("ShareURL is not set", func() {
 			BeforeEach(func() {
 				conf.Server.ShareURL = ""