fix(flatfile): page-up cursor must not stick on entries[0]

When the cursor lands on entries[0].byte_offset, the !from_start
backup logic produces an empty [X, X) range, get_previous_chat
returns DB_RESPONSE_EMPTY, the UI sets WIN_SCROLL_REACHED_TOP and
never tries again — page-up is permanently stuck.

Guard the cursor optimisation with two conditions: index must be
non-empty AND cursor must not equal entries[0].byte_offset. When
either fails, fall through to the end_time bisect path which
correctly locates the byte range with older messages.

F16 regression test in bench_failure_modes:
  without guard: received=200 in 3 iters (premature EMPTY)
  with guard:    received=600 in 7 iters (truly reaches top)

src/database_flatfile.c

@@ -728,8 +728,13 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
     off_t read_from = state->bom_len;
     off_t read_to = state->stamp.size;
 
-    // Use stored cursor for sequential Page Up (skip bisect)
-    if (!start_time && end_time && state->cursor_offset >= 0) {
+    // Use stored cursor for sequential Page Up (skip bisect).
+    // Guard: when cursor lands on entries[0].byte_offset, the !from_start
+    // backup below produces an empty [X, X) range; falling back to the
+    // end_time bisect avoids permanently sticking page-up at the top.
+    if (!start_time && end_time && state->cursor_offset >= 0
+        && state->n_entries > 0
+        && state->cursor_offset != state->entries[0].byte_offset) {
         read_to = state->cursor_offset;
     } else if (end_time) {
         // Upper-bound: find first index entry AFTER end_time