Fix: transfer expiry date picker now uses the correct time

Problem: When selecting an expiry date, the transfer expired at 00:00
of the selected day (the very start), not the end. Selecting "March 17" would
expire the transfer at midnight between March 16 and March 17 — potentially
just hours after creation.

Solution: The expiry timestamp is now set to the current time-of-day
on the selected date. If you upload at 14:32 and select "March 17", the
transfer expires on March 17 at 14:32 — exactly one or more full days later.
Selecting today as the expiry date is not allowed.
Additionally, all templates that display the expiry date now show the full
datetime (date + time), so users can see the exact expiry moment.

Note: For users to see the expiry time in their own timezone rather than
the server's UTC time, the option client_send_current_timezone_to_server
should be set to true in config.php. This option is false by default
for privacy reasons (it sends the browser's timezone as a cookie), but
enabling it is recommended for deployments where all users share the same
timezone or where showing the correct local time is a priority. Without it,
the expiry time is displayed in UTC, which may confuse users in other
timezones.

Files modified:

• www/js/upload_page.js — expiry timestamp calculation
• templates/transfers_table.php
• templates/transfer_detail_page.php
• templates/transfers_guest_page.php
• templates/download_page.php