fix(tui): Ctrl+C with input selection actually preserves input (lift handler to app level)

Previous fix in 9dbf1ec6 handled Ctrl+C inside textInput but the APP-level
useInputHandlers fires the same keypress in a separate React hook and ran
clearIn() regardless. Net effect: the OSC 52 copy succeeded but the input
wiped right after, so Brooklyn only noticed the wipe.

Lift the selection-aware Ctrl+C to a single place by threading input
selection state through a new nanostore (src/app/inputSelectionStore.ts).
textInput syncs its derived `selected` range + a clear() callback to the
store on every selection change, and the app-level Ctrl+C handler reads
the store before its clear/interrupt/die chain:

  - terminal-level selection (scrollback) → copy, existing behavior
  - in-input selection present → copy + clear selection, preserve input
  - input has text, no selection → clearIn(), existing behavior
  - empty + busy → interrupt turn
  - empty + idle → die

textInput no longer has its own Ctrl+C block; keypress falls through to
app-level like it did before 9dbf1ec6.

ui-tui/src/app/inputSelectionStore.ts

@@ -0,0 +1,14 @@
+import { atom } from 'nanostores'
+
+export interface InputSelection {
+  clear: () => void
+  end: number
+  start: number
+  value: string
+}
+
+export const $inputSelection = atom<InputSelection | null>(null)
+
+export const setInputSelection = (next: InputSelection | null) => $inputSelection.set(next)
+
+export const getInputSelection = () => $inputSelection.get()