## Description
The widget email-transcript button (`ChatFooter.vue`) had no client-side
guard. Its visibility depends only on whether the contact has an email,
and the click handler fired a request on every click with no in-flight
lock, no disabled state, and no post-send handling. A user could
therefore trigger a large number of duplicate transcript emails from a
single conversation just by clicking repeatedly.
This adds a re-entry guard in the handler, disables the button while a
send is in flight, and applies a short cooldown (15s) after a successful
send. Normal use is unaffected: the button sends once, shows the success
toast, then briefly disables and automatically re-enables so a genuine
later re-request still works. On failure the button stays enabled so the
user can retry immediately. The cooldown timer is cleared on unmount.
Using a timed cooldown (rather than a permanent post-send lock) also
avoids the button getting stuck disabled if a resolved conversation is
reopened and later re-resolved.
This is the client-side complement to the server-side rate limit added
in #15085.
Fixes https://linear.app/chatwoot/issue/CW-7640
