v0.15.0 — selectable timezone

The bot's decision slots are no longer hard-wired to Irish time. Set TIMEZONE (an IANA name like America/New_York, default Europe/Dublin) in .env or on the settings page, and the daily 06:00, Monday and 1st-of-month decision slots run on your local clock — line it up with the schedule you set. Validated on save; safe to change anytime (no money implication, so it isn't locked like the base currency). Default stays Europe/Dublin, so existing installs are unaffected. Closes #24.