fix: Finnish lang typos and small rewording

[ikke-t]

I did not add or remove translations, just reworded some and fixed typos.

[Copilot]

Pull request overview

This PR refines Finnish localization copy by correcting typos and rewording a few labels/messages to be clearer and more consistent with “calendar” wording across the UI/email templates.

Changes:

• Reworded the iCal link text in Finnish reservation emails from “Lisää Outlookiin” to “Lisää kalenteriisi”.
• Fixed multiple Finnish typos and improved phrasing in lang/fi_fi.php (e.g., “sähköpostiosoite”, “Pahoittelut”, “Kutsu Käyttäjiä”).
• Updated a few admin/email subject strings to use Finnish (“Ilmoitus”) instead of English (“Notification”).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lang/fi_fi/ReservationInvitation.tpl	Updates Finnish email CTA for adding the reservation to a calendar.
lang/fi_fi/ReservationCreated.tpl	Updates Finnish email CTA for adding the reservation to a calendar.
lang/fi_fi.php	Fixes Finnish typos and improves phrasing for several UI and email strings.