fix: use \033[2K to fully clear spinner lines and prevent text remnants

Fixes text remnants and extra blank lines when spinner messages change.

Issues fixed:
1. Text remnants when switching from longer to shorter messages (e.g., 'Cleaning...ems...')
2. Extra blank lines appearing after section headers

Root causes:
- \033[K only clears from cursor to end of line, leaving remnants when new messages are shorter
- stop_section_spinner was clearing lines even when no spinner was running

Changes:
- lib/core/base.sh:
  - Changed stop_section_spinner(), safe_clear_line(), safe_clear_lines() to use \033[2K
  - Added guard in stop_section_spinner to only clear when spinner is actually running
- lib/core/ui.sh:
  - Clear line once when starting spinner (before loop) to ensure clean start
  - Normal spinner rotation uses \r without clearing (performance optimization)

Performance: Line clearing happens only once per spinner start, not on every loop iteration.

Fixes #390

bin/clean.sh

@@ -142,10 +142,6 @@ cleanup() {
 
     stop_inline_spinner 2> /dev/null || true
 
-    if [[ -t 1 ]]; then
-        printf "\r\033[K" >&2 || true
-    fi
-
     cleanup_temp_files
 
     stop_sudo_session