perf(purge/uninstall): eliminate per-item subprocess forks in scan phase

purge:
- is_safe_project_artifact: replace echo|tr|wc depth calc with pure bash
  string arithmetic (2 forks → 0 per item)
- process_scan_results: replace dirname subprocess with ${item%/*}
- is_recently_modified: accept pre-computed epoch to avoid redundant
  get_epoch_seconds call (N date forks → 1 before loop)
- fd pattern construction: replace 45 per-target sed forks with single
  printf|sed pass (45 forks → 2)
- pre-compute _cached_project_paths alongside existing name/basename
  cache so display loop avoids get_project_path subshell per item

uninstall/app-selector:
- default size display: N/A → -- for apps without cached size data
  (cleaner UX on first scan; real sizes populate from background refresh)
- Pass 1 scan: replace basename/dirname subshells with parameter expansion

lib/ui/app_selector.sh

@@ -17,8 +17,8 @@ format_app_display() {
     fi
 
     # Format size
-    local size_str="N/A"
-    [[ "$size" != "0" && "$size" != "" && "$size" != "Unknown" ]] && size_str="$size"
+    local size_str="--"
+    [[ "$size" != "0" && "$size" != "" && "$size" != "Unknown" && "$size" != "N/A" && "$size" != "--" ]] && size_str="$size"
 
     # Calculate available width for app name based on terminal width
     # Accept pre-calculated max_name_width (5th param) to avoid recalculation in loops