fix(window): preserve scroll offset on page_down and refactor

When paging down beyond the visible chat buffer, the scroll position could
jump unexpectedly. This occurred because the visual distance from the bottom
(e.g. 6 lines until the last message) wasn’t preserved after loading new
messages from the DB.

This patch fixes that by capturing the visual offset (`current_offset`) before
the DB fetch and reapplying it after, based on the y-position of the final
buffer entry. This ensures consistent and smooth scroll transitions, even
as message heights vary.

In addition to fixing the visual glitch, this commit introduces a targeted
performance optimization: once the bottom of the chat has been reached
(`WIN_SCROLL_REACHED_BOTTOM`), we skip expensive operations entirely —
including datetime lookups and entry formatting — during further page_down
events. This reduces performance stalls
during rapid paging (e.g. holding the Page Down key at the bottom of the scrolling area).

This update also adapts to the recent change in `chatwin_db_history()`,
which now returns a `db_history_result_t` instead of `gboolean`, allowing
correct differentiation between an empty result and an actual DB error.

Together, these changes:
- Fix a scroll-jump bug in `page_down`
- Improve performance and responsiveness during rapid navigation
- Align with improved DB result handling

src/ui/window.c

@@ -710,17 +710,21 @@ win_page_down(ProfWin* window, int scroll_size)
     // Scrolled down after reaching the bottom of the page
     if ((*page_start > total_rows - page_space || (*page_start == page_space && *page_start >= total_rows)) && window->type == WIN_CHAT) {
         int bf_size = buffer_size(window->layout->buffer);
-        if (bf_size > 0) {
+        if (bf_size > 0 && *scroll_state != WIN_SCROLL_REACHED_BOTTOM) {
+            // How many lines are left until end of the screen
+            int current_offset = total_rows - *page_start;
             GDateTime* now = g_date_time_new_now_local();
             ProfBuffEntry* last_entry = buffer_get_entry(window->layout->buffer, bf_size - 1);
             auto_gchar gchar* start = g_date_time_format_iso8601(last_entry->time);
             auto_gchar gchar* end_date = g_date_time_format_iso8601(now);
-            if (*scroll_state != WIN_SCROLL_REACHED_BOTTOM && !chatwin_db_history((ProfChatWin*)window, start, end_date, FALSE)) {
+            db_history_result_t db_response = chatwin_db_history((ProfChatWin*)window, start, end_date, FALSE);
+            if (db_response == DB_RESPONSE_EMPTY)
                 *scroll_state = WIN_SCROLL_REACHED_BOTTOM;
-            }
 
-            int offset = last_entry->y_end_pos - 1;
-            *page_start = offset - page_space + scroll_size;
+            // similar to page_up (see explanation there)
+            // Get offset of the message that was the latest prior to loading messages from the DB
+            int original_pos = last_entry->y_end_pos;
+            *page_start = original_pos - current_offset;
             g_date_time_unref(now);
         }
     }