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/ui/chatwin.c

@@ -561,7 +561,8 @@ static void
 _chatwin_history(ProfChatWin* chatwin, const char* const contact_barejid)
 {
     if (!chatwin->history_shown) {
-        GSList* history = log_database_get_previous_chat(contact_barejid, NULL, NULL, FALSE, FALSE);
+        GSList* history = NULL;
+        log_database_get_previous_chat(contact_barejid, NULL, NULL, FALSE, FALSE, &history);
         GSList* curr = history;
 
         while (curr) {
@@ -583,15 +584,15 @@ _chatwin_history(ProfChatWin* chatwin, const char* const contact_barejid)
 // Print history starting from start_time to end_time if end_time is null the
 // first entry's timestamp in the buffer is used. Flip true to prepend to buffer.
 // Timestamps should be in iso8601
-gboolean
+db_history_result_t
 chatwin_db_history(ProfChatWin* chatwin, const char* start_time, char* end_time, gboolean flip)
 {
     if (!end_time) {
         end_time = buffer_size(((ProfWin*)chatwin)->layout->buffer) == 0 ? NULL : g_date_time_format_iso8601(buffer_get_entry(((ProfWin*)chatwin)->layout->buffer, 0)->time);
     }
 
-    GSList* history = log_database_get_previous_chat(chatwin->barejid, start_time, end_time, !flip, flip);
-    gboolean has_items = g_slist_length(history) != 0;
+    GSList* history = NULL;
+    db_history_result_t result = log_database_get_previous_chat(chatwin->barejid, start_time, end_time, !flip, flip, &history);
     GSList* curr = history;
 
     while (curr) {
@@ -610,9 +611,12 @@ chatwin_db_history(ProfChatWin* chatwin, const char* start_time, char* end_time,
     }
 
     g_slist_free_full(history, (GDestroyNotify)message_free);
+    // TODO: Potential optimization
+    // if (flip)
+    // since adding messages to history without overflowing buffer does not require a win_redraw
     win_redraw((ProfWin*)chatwin);
 
-    return has_items;
+    return result;
 }
 
 static void