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/ui.h

@@ -43,6 +43,7 @@
 #include "config/account.h"
 #include "config/preferences.h"
 #include "command/cmd_funcs.h"
+#include "database.h"
 #include "ui/win_types.h"
 #include "xmpp/message.h"
 #include "xmpp/muc.h"
@@ -145,7 +146,7 @@ void chatwin_set_incoming_char(ProfChatWin* chatwin, const char* const ch);
 void chatwin_unset_incoming_char(ProfChatWin* chatwin);
 void chatwin_set_outgoing_char(ProfChatWin* chatwin, const char* const ch);
 void chatwin_unset_outgoing_char(ProfChatWin* chatwin);
-gboolean chatwin_db_history(ProfChatWin* chatwin, const char* start_time, char* end_time, gboolean flip);
+db_history_result_t chatwin_db_history(ProfChatWin* chatwin, const char* start_time, char* end_time, gboolean flip);
 
 // MUC window
 ProfMucWin* mucwin_new(const char* const barejid);