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.h

@@ -40,14 +40,20 @@
 #include "config/account.h"
 #include "xmpp/xmpp.h"
 
-#define MESSAGES_TO_RETRIEVE 10
+#define MESSAGES_TO_RETRIEVE 100
+
+typedef enum {
+    DB_RESPONSE_ERROR,
+    DB_RESPONSE_EMPTY,
+    DB_RESPONSE_SUCCESS
+} db_history_result_t;
 
 gboolean log_database_init(ProfAccount* account);
 void log_database_add_incoming(ProfMessage* message);
 void log_database_add_outgoing_chat(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
 void log_database_add_outgoing_muc(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
 void log_database_add_outgoing_muc_pm(const char* const id, const char* const barejid, const char* const message, const char* const replace_id, prof_enc_t enc);
-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);
 ProfMessage* log_database_get_limits_info(const gchar* const contact_barejid, gboolean is_last);
 void log_database_close(void);