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/gatewayTypes.ts

@@ -82,7 +82,7 @@ export interface ConfigVoiceConfig {
 }
 
 export interface ConfigFullResponse {
-  config?: { display?: ConfigDisplayConfig; voice?: ConfigVoiceConfig }
+  config?: { display?: ConfigDisplayConfig; voice?: ConfigVoiceConfig; paste_collapse_threshold?: number }
 }
 
 export interface ConfigMtimeResponse {