fix(x-ui.sh): address review feedback on migrateDB

Per Copilot review on PR #4910:
- Probe the bundled binary for migrate-db --dump support and fail with a clear
  upgrade message instead of a raw "flag not defined" error on old builds.
- Prompt before overwriting an existing .dump in dump mode (parity with restore).
- Refuse to restore into the live database path while x-ui is running, to avoid
  corrupting the running panel.
- Fix the usage/synopsis strings to show input is optional ([file] not <file>).

x-ui.sh

@@ -2806,6 +2806,10 @@ postgresql_menu() {
     esac
 }
 
+# Convert between the panel's SQLite database and a portable .dump (SQL text)
+# file using the bundled x-ui binary. With no arguments it dumps the installed
+# panel database; an optional second argument overrides the output path.
+#   x-ui migrateDB [file.db|file.dump] [output]
 migrate_db() {
     local input="$1" output="$2"
     local default_db="/etc/x-ui/x-ui.db"
@@ -2818,9 +2822,15 @@ migrate_db() {
         return 1
     fi
 
+    if ! "$bin" migrate-db -h 2>&1 | grep -q -- '-dump'; then
+        LOGE "This x-ui build does not support .db <-> .dump conversion yet."
+        LOGE "Update the panel first (x-ui update) to a version with 'migrate-db --dump/--restore'."
+        return 1
+    fi
+
     if [[ ! -f "$input" ]]; then
         LOGE "Input file not found: ${input}"
-        echo -e "Usage: ${green}x-ui migrateDB <file.db|file.dump> [output]${plain}"
+        echo -e "Usage: ${green}x-ui migrateDB [file.db|file.dump] [output]${plain}"
         return 1
     fi
 
@@ -2843,6 +2853,9 @@ migrate_db() {
 
     if [[ "$mode" == "dump" ]]; then
         [[ -z "$output" ]] && output="${input%.*}.dump"
+        if [[ -f "$output" ]]; then
+            confirm "Output ${output} already exists and will be overwritten. Continue?" "n" || return 0
+        fi
         LOGI "Dumping SQLite database to SQL text:"
         echo -e "  ${green}${input}${plain} -> ${green}${output}${plain}"
         if "$bin" migrate-db --src "$input" --dump "$output"; then
@@ -2853,6 +2866,11 @@ migrate_db() {
         fi
     else
         [[ -z "$output" ]] && output="${input%.*}.db"
+        if [[ "$output" == "$default_db" ]] && check_status > /dev/null 2>&1; then
+            LOGE "Refusing to restore into the live database (${default_db}) while x-ui is running."
+            LOGE "Stop the panel first (x-ui stop) or choose a different output path."
+            return 1
+        fi
         if [[ -f "$output" ]]; then
             confirm "Output ${output} already exists and will be overwritten. Continue?" "n" || return 0
             rm -f "$output"
@@ -2898,7 +2916,7 @@ show_usage() {
 │  ${blue}x-ui banlog${plain}                - Check Fail2ban ban logs          │
 │  ${blue}x-ui update${plain}                - Update                           │
 │  ${blue}x-ui update-all-geofiles${plain}   - Update all geo files             │
-│  ${blue}x-ui migrateDB <file>${plain}      - Convert .db <-> .dump (SQLite)   │
+│  ${blue}x-ui migrateDB [file]${plain}      - Convert .db <-> .dump (SQLite)   │
 │  ${blue}x-ui legacy${plain}                - Legacy version                   │
 │  ${blue}x-ui install${plain}               - Install                          │
 │  ${blue}x-ui uninstall${plain}             - Uninstall                        │