style: normalize whitespace and line endings in shell scripts

- Standardize indentation and spacing across install.sh and lib files
- Fix line ending inconsistencies for better cross-platform compatibility
- Improve curl flag usage in neovim.sh (-kLo with explicit temp path)
- Add theme persistence functionality to remember user's colorscheme choice

These changes improve code consistency and add the ability to save/restore
the selected theme across sessions.

install.sh

@@ -53,12 +53,17 @@ sudo -v || error "Failed to obtain sudo. Aborting."
 ( while kill -0 $$ 2>/dev/null; do sudo -n true; sleep 50; done ) &
 SUDO_KEEPALIVE_PID=$!
 
+INSTALL_SUCCESS=false
+
 cleanup() {
+  local exit_code=$?
   kill $SUDO_KEEPALIVE_PID 2>/dev/null
   if ! $VERBOSE && [[ -e /proc/self/fd/3 ]]; then
     exec 1>&3 2>&4 3>&- 4>&-
-    echo ""
-    echo -e "  ${RED}[✗]${NC} Installation failed. See log: $LOG_FILE"
+    if ! $INSTALL_SUCCESS; then
+      echo ""
+      echo -e "  ${RED}[✗]${NC} Installation failed (exit $exit_code). See log: $LOG_FILE"
+    fi
   fi
 }
 trap cleanup EXIT
@@ -84,6 +89,8 @@ if ! $VERBOSE; then
   end_quiet_mode
 fi
 
+INSTALL_SUCCESS=true
+
 # ── Done ────────────────────────────────────────────────────────
 echo ""
 echo "  ╔═══════════════════════════════════════╗"