feat: configurable paste collapse thresholds (TUI + CLI)

Adds two new config keys:
- paste_collapse_threshold (default: 5) — line count threshold for
  bracketed paste collapse in both TUI and CLI
- paste_collapse_threshold_fallback (default: 0, disabled) — same for
  the fallback heuristic in terminals without bracketed paste support

TUI frontend reads these from config.get full via applyDisplay/patchUiState.
CLI reads from self.config at paste-handling time.

Closes #5626
Related: #5623

ui-tui/src/app/interfaces.ts

@@ -105,6 +105,8 @@ export interface UiState {
   info: null | SessionInfo
   inlineDiffs: boolean
   mouseTracking: MouseTrackingMode
+  pasteCollapseLines: number
+
   sections: SectionVisibility
   showCost: boolean
   showReasoning: boolean