httpd: give the session cookie a Max-Age matching the timeout

The session cookie carried neither Expires nor Max-Age, so browsers that
clean up session cookies aggressively (LibreWolf, for one) may drop it
while the session is still valid. Send Max-Age=session_timeout so the
declared lifetime matches the timeout already enforced by the firmware.

itoa16_html() only converted four digits, so extend it to five to print
timeouts up to 65535. That also fixes information.json, which used the
same helper and reported wrong values from 10000 s up.

httpd/page_impl.h

@@ -5,6 +5,8 @@
 
 extern __code uint8_t * __code HTTP_RESPONCE_TXT;
 
+void itoa16_html(uint16_t v);
+
 bool send_counters(uint8_t phys_port);
 void send_status(void);
 void send_vlan(uint16_t vlan);