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/uiStore.ts

@@ -17,6 +17,7 @@ const buildUiState = (): UiState => ({
   info: null,
   inlineDiffs: true,
   mouseTracking: MOUSE_TRACKING,
+  pasteCollapseLines: 5,
   sections: {},
   showCost: false,
   showReasoning: false,