refactor(tui): statusbar as 4-mode position (on|off|bottom|top)

Default is back to 'on' (inline, above the input) — bottom was too far
from the input and felt disconnected. Users who want it pinned can
opt in explicitly.

- UiState.statusBar: boolean → 'on' | 'off' | 'bottom' | 'top'
- /statusbar [on|off|bottom|top|toggle]; no-arg still binary-toggles
  between off and on (preserves muscle memory)
- appLayout renders StatusRulePane in three slots (inline inside
  ComposerPane for 'on', above transcript row for 'top', after
  ComposerPane for 'bottom'); only the slot matching ui.statusBar
  actually mounts
- drop the input's marginBottom when 'bottom' so the rule sits tight
  against the input instead of floating a row below
- useConfigSync.normalizeStatusBar coerces legacy bool (true→on,
  false→off) and unknown shapes to 'on' for forward-compat reads
- tui_gateway: split compact from statusbar config handlers; persist
  string enum with _coerce_statusbar helper for legacy bool configs

ui-tui/src/app/interfaces.ts

@@ -27,6 +27,8 @@ export interface StateSetter<T> {
   (value: SetStateAction<T>): void
 }
 
+export type StatusBarMode = 'bottom' | 'off' | 'on' | 'top'
+
 export interface SelectionApi {
   clearSelection: () => void
   copySelection: () => string
@@ -89,7 +91,7 @@ export interface UiState {
   showReasoning: boolean
   sid: null | string
   status: string
-  statusBar: boolean
+  statusBar: StatusBarMode
   streaming: boolean
   theme: Theme
   usage: Usage