ui: Filesystem @mentions for Chat Form (#26715)

* base : @-mention picker foundation - glob search, picker nav, highlight

* feat : @-mention file/folder picker and mention badges in message bubbles

* fix: Imports

* feat : wire the @-mention picker into the chat form

* fix: Bound the glob-search result cache key and prune stale entries

tools/ui/src/lib/enums/keyboard.enums.ts

@@ -8,6 +8,7 @@ export enum KeyboardKey {
 	ARROW_DOWN = 'ArrowDown',
 	ARROW_LEFT = 'ArrowLeft',
 	ARROW_RIGHT = 'ArrowRight',
+	BACKSPACE = 'Backspace',
 	TAB = 'Tab',
 	B_LOWER = 'b',
 	D_LOWER = 'd',