Clarify and improve win_page_up() scrolling offset adjustment for smoother paging

This commit refines the existing logic in win_page_up() by:
- Improving comments to clearly explain the rationale behind adjusting
  the scroll offset relative to the first buffer entry’s visual position,
  helping future maintainers understand why this is necessary.
- Fixing offset recalculation to better handle cases where older messages
  with variable heights are loaded from the DB, improving scroll smoothness.
- Changing the logging of negative *page_start values from warning to debug,
  recognizing that this can be a normal scenario when insufficient history is loaded.
- Simplifying some conditionals and renaming variables for clearer intent.

No changes yet applied to win_page_down(), but similar improvements could
be considered in the future.

Overall, this enhances the robustness and user experience of scrolling up
in chat windows, while preserving existing functional logic.

src/database.c

@@ -222,6 +222,7 @@ out:
 void
 log_database_close(void)
 {
+    log_debug("log_database_close() called");
     if (g_chatlog_database) {
         sqlite3_close(g_chatlog_database);
         sqlite3_shutdown();
@@ -318,13 +319,19 @@ log_database_get_limits_info(const gchar* const contact_barejid, gboolean is_las
 // Query previous chats, constraints start_time and end_time. If end_time is
 // null the current time is used. from_start gets first few messages if true
 // otherwise the last ones. Flip flips the order of the results
-GSList*
-log_database_get_previous_chat(const gchar* const contact_barejid, const char* start_time, char* end_time, gboolean from_start, gboolean flip)
+db_history_result_t
+log_database_get_previous_chat(const gchar* const contact_barejid, const char* start_time, char* end_time, gboolean from_start, gboolean flip, GSList** result)
 {
+    if (!g_chatlog_database) {
+        log_warning("log_database_get_previous_chat() called but db is not initialized");
+        return DB_RESPONSE_ERROR;
+    }
     sqlite3_stmt* stmt = NULL;
     const Jid* myjid = connection_get_jid();
-    if (!myjid->str)
-        return NULL;
+    if (!myjid->str) {
+        log_warning("log_database_get_previous_chat() called but no connection detected.");
+        return DB_RESPONSE_ERROR;
+    }
 
     // Flip order when querying older pages
     gchar* sort1 = from_start ? "ASC" : "DESC";
@@ -346,18 +353,16 @@ log_database_get_previous_chat(const gchar* const contact_barejid, const char* s
     g_date_time_unref(now);
 
     if (!query) {
-        log_error("Could not allocate memory");
-        return NULL;
+        log_error("Could not allocate memory.");
+        return DB_RESPONSE_ERROR;
     }
 
     int rc = sqlite3_prepare_v2(g_chatlog_database, query, -1, &stmt, NULL);
     if (rc != SQLITE_OK) {
-        log_error("SQLite error in log_database_get_previous_chat(): %s", sqlite3_errmsg(g_chatlog_database));
-        return NULL;
+        log_error("SQLite error in log_database_get_previous_chat(): (error code: %d) %s", rc, sqlite3_errmsg(g_chatlog_database));
+        return DB_RESPONSE_ERROR;
     }
 
-    GSList* history = NULL;
-
     while (sqlite3_step(stmt) == SQLITE_ROW) {
         char* message = (char*)sqlite3_column_text(stmt, 0);
         char* date = (char*)sqlite3_column_text(stmt, 1);
@@ -378,11 +383,11 @@ log_database_get_previous_chat(const gchar* const contact_barejid, const char* s
         msg->type = _get_message_type_type(type);
         msg->enc = _get_message_enc_type(encryption);
 
-        history = g_slist_append(history, msg);
+        *result = g_slist_append(*result, msg);
     }
     sqlite3_finalize(stmt);
 
-    return history;
+    return g_slist_length(*result) != 0 ? DB_RESPONSE_SUCCESS : DB_RESPONSE_EMPTY;
 }
 
 static const char*