Fix prompt() silently truncating multi-line pasted input

`prompt()` used a single `read -r`, which stops at the first newline.
If a pasted value (most notably join-data copied from the panel UI)
arrives with an embedded newline, everything after it was silently
dropped, and the leftover fragment would be consumed by the *next*
prompt instead. The truncated-but-still-valid-looking base64 could
still decode successfully, just missing trailing fields like `remote`,
producing a config.yml that fails at startup with "invalid remote
configuration, cannot connect to panel".

prompt() now drains any additional lines already buffered on the TTY
after the first read, so a multi-line paste is captured in full
without blocking on further keystrokes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

install-wings.sh

@@ -34,9 +34,19 @@ need_tty() {
 
 prompt() {
   local msg="$1"
-  local out
+  local out line
   printf "${BOLD}%s${RESET}" "$msg" >"$TTY"
   IFS= read -r out <"$TTY"
+
+  # A pasted value (e.g. join-data copied from the panel UI) can arrive with an
+  # embedded newline. `read` stops at the first one, which would otherwise
+  # silently truncate the rest onto a "phantom" line consumed by a later
+  # prompt. Drain any additional lines that are already buffered so nothing
+  # gets lost; this never blocks waiting on new keystrokes.
+  while IFS= read -r -t 0.1 line <"$TTY" 2>/dev/null; do
+    out+="$line"
+  done
+
   printf "%s" "$out"
 }