Add spinner/countdown-bar to stats panel; backfill on widened days_back

Stats panel: a persistent spinner marks whichever target is currently
being scanned/reacted to, and jittered waits show a live-ticking
countdown bar instead of a static "waiting Xs" line. Both the status
line and countdown row are always rendered (blank when idle) so their
appearing/disappearing no longer shifts the table's height.

Backlog: channel_scan_state now tracks oldest_covered_at alongside
newest_id_seen. If days_back is widened after a channel already has a
resume checkpoint, the forward-only diff would never look back far
enough to notice — now the newly-exposed gap gets backfilled first.

bot/undo.py

@@ -25,6 +25,7 @@ async def undo_all_reactions(client: discord.Client, storage: Storage, display:
         should_clear_log = True
 
         if display is not None:
+            display.set_active_target(row["target_id"])
             display.set_status(f"Removing reaction from message {message_id} ({i}/{total})...")
 
         try:
@@ -46,10 +47,13 @@ async def undo_all_reactions(client: discord.Client, storage: Storage, display:
 
         delay = human_jitter()
         if display is not None:
-            display.set_status(f"Waiting {delay:.1f}s before next removal...")
+            display.start_countdown(delay, "next removal")
         await asyncio.sleep(delay)
+        if display is not None:
+            display.clear_countdown()
 
     if display is not None:
+        display.set_active_target(None)
         display.set_status("Clearing channel resume points and found-message log...")
     cleared = await asyncio.to_thread(storage.clear_scan_state)
     await asyncio.to_thread(storage.reset_message_counts)