feat: add stacking history with fading older entries

The last historyCount combos (1-5, default 1) stack on screen instead
of vanishing: each new combo pushes the previous one down the stack and
every older entry fades step by step (entryOpacity gradient). The stack
direction follows the position by default — top positions grow downward
(newest on top), bottom positions grow upward (newest at the bottom
edge) — and is not configurable. The panel gains a History NumberField
for the count.

Middle positions (center-left/center-right/center-center) were removed
from the dropdown and any leftover config value is folded into the
bottom row. With lingerMs 0 entries never expire, so the stack keeps
the last few combos on screen until newer ones push them out.

Manifest bumped to 1.6.0.

manifest.json

@@ -2,7 +2,7 @@
   "schemaVersion": 1,
   "id": "felixzsh.key-visualizer",
   "name": "Key Visualizer",
-  "version": "1.5.0",
+  "version": "1.6.0",
   "author": "felixzsh",
   "description": "Shows the keys you press on screen. Great for keybinding tutorials, demos, and screencasts.",
   "kinds": [