refactor(reorder): drop the Alt + Arrow shortcut, keep drag only

The reorder shortcut cannot be made reliable in this UI. `Alt + Arrow` is
the browser's tab-history / back-forward gesture on several platforms,
and the modal already binds bare `ArrowLeft`/`ArrowRight` to model
navigation, so the binding either did nothing — a keypress with nothing
focused never reaches a listener on the tag list — or fought the browser.
An affordance that occasionally navigates the page away is worse than
having no keyboard path at all, so drop it.

Reordering is pointer-only again: drag the chip (tags) or its `⠿` grip
(trigger words, whose chip body is click-to-edit). Everything that existed
only to serve the shortcut goes with it — the keydown listener, the hover
tracking used to resolve the target chip, the aria-live announcements, the
per-grip position labels and `moveItemWithinContainer`. The grip becomes a
decorative, non-focusable `<span>` (`aria-hidden`, behind a 5px drag
threshold) instead of a `<button>`, so it no longer promises a keyboard
action it cannot perform.

The tooltip and hint drop the shortcut mention in all 10 locales
(`common.reorder.dragHandle` = "Drag to reorder" and the localised
equivalents); `common.reorder.ariaLabel` and `common.reorder.announcement`
are pruned from every locale by the sync script. The i18n guidelines
record the decision so no shortcut is re-added without re-adding the keys.

locales/zh-TW.json

@@ -3,9 +3,7 @@
         "cancel": "取消",
         "confirm": "確認",
         "reorder": {
-            "dragHandle": "拖曳以調整順序（Alt + ↑/↓）",
-            "ariaLabel": "調整 {item} 的順序，第 {position} 個，共 {total} 個",
-            "announcement": "已移動到第 {position} 個，共 {total} 個"
+            "dragHandle": "拖曳以調整順序"
         },
         "actions": {
             "save": "儲存",