multimedia/youtui: fix build on armv7

The usual issue of time_t being 64 bits on armv7 despite it being a
32 bit platform. Refactor the time conversion code to just use i64
throughout, avoiding any type conversion issues.

Approved by: portmgr (build fix blanket)
See also: 0c9905d54dedcf8763df802a01beb2ea3d4ac4a0
MFH: 2026Q2

(cherry picked from commit 937f50b1ebc08224182b644e0fc2c45668e2a0cb)