Fix stats off-by-an-hour by reading the site's named timezone (#23124)

The site-info endpoint returns the Olson timezone under `options.timezone`,
not `timezone_string`, so the previous fix silently fell back to the stale
numeric `gmt_offset`, shifting the stats "day" boundary by an hour across DST.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: adalpari

libs/fluxc/src/main/java/org/wordpress/android/fluxc/network/rest/wpcom/site/SiteRestClient.kt

@@ -1087,8 +1087,11 @@ class SiteRestClient @Inject constructor(
             site.hasWooCommerce = from.options.woocommerce_is_active
             site.adminUrl = from.options.admin_url
             site.loginUrl = from.options.login_url
-            site.timezone = if (!from.options.timezone_string.isNullOrEmpty()) {
-                from.options.timezone_string
+            // Prefer the named Olson timezone (e.g. "Europe/Madrid") over the numeric gmt_offset so
+            // DST is handled correctly. The numeric offset reflects only the offset at fetch time and
+            // goes stale across DST transitions, which shifts the stats "day" boundary by an hour.
+            site.timezone = if (!from.options.timezone.isNullOrEmpty()) {
+                from.options.timezone
             } else {
                 from.options.gmt_offset
             }

libs/fluxc/src/main/java/org/wordpress/android/fluxc/network/rest/wpcom/site/SiteWPComRestResponse.java

@@ -23,7 +23,9 @@ public static class Options {
         public String admin_url;
         public String login_url;
         public String gmt_offset;
-        public String timezone_string;
+        // The site info endpoints (/me/sites, /sites/$id) expose the Olson timezone under "timezone"
+        // (e.g. "Europe/Madrid"). Note the site *settings* endpoint uses "timezone_string" instead.
+        public String timezone;
         public String frame_nonce;
         public String unmapped_url;
         public String max_upload_size;

libs/fluxc/src/test/java/org/wordpress/android/fluxc/network/rest/wpcom/site/SiteRestClientTest.kt

@@ -98,6 +98,38 @@ class SiteRestClientTest {
         )
     }
 
+    @Test
+    fun `maps named Olson timezone from options timezone field`() = test {
+        val response = SiteWPComRestResponse()
+        response.ID = siteId
+        response.URL = "site.com"
+        response.options = SiteWPComRestResponse.Options().apply {
+            timezone = "Europe/Madrid"
+            gmt_offset = "1"
+        }
+
+        initSiteResponse(response)
+
+        val responseModel = restClient.fetchSite(site)
+        assertThat(responseModel.timezone).isEqualTo("Europe/Madrid")
+    }
+
+    @Test
+    fun `falls back to gmt_offset when options timezone is empty`() = test {
+        val response = SiteWPComRestResponse()
+        response.ID = siteId
+        response.URL = "site.com"
+        response.options = SiteWPComRestResponse.Options().apply {
+            timezone = ""
+            gmt_offset = "-5"
+        }
+
+        initSiteResponse(response)
+
+        val responseModel = restClient.fetchSite(site)
+        assertThat(responseModel.timezone).isEqualTo("-5")
+    }
+
     @Test
     fun `fetchSite returns error when API call fails`() = test {
         val errorMessage = "message"