docs: add Ukrainian quick-start README

[slegarraga]

Part of #16.

Adds README.uk.md following the short quick-start shape established by README.es.md:

• stable content only (what the app does, install table, per-OS permission notes, honest caveats)
• <ver> placeholders instead of hardcoded versions
• link added under the intro in the main README.md

The Spanish half of #16 was already merged; this covers the Ukrainian half.

[vstrelnikof]

Merged — thank you, and thank you for closing the other half of #16 rather than stopping at the language you speak.

The shape is exactly what #24 established: stable content only, <ver> placeholders so a release cannot make it stale, the language link folded into the one line under the intro. I checked the same things as last time — the four installer filenames against the real release assets, every link relative and resolving, no hardcoded versions anywhere. The Ukrainian typography is right too, which is the part machine translation usually gets wrong: ’ (U+2019) rather than a straight apostrophe, «» rather than "".

One thing I fixed on top rather than sending back to you: the Flatpak paragraph lost its link to docs/DECISIONS.md. Not your oversight — README.es.md had the same gap when you copied its shape, and I only fixed it there a few hours after you opened this. A reader who disagrees with the "no Flatpak" decision should be able to find the argument, so it is back in both files now.

Two UI strings a native speaker should confirm before we call this final. I have flagged them to the maintainer rather than guessing:

• Windows SmartScreen. The table says «Windows захистив ваш ПК» → Додатково → Виконати все одно. Ukrainian Windows may word these differently. If it does, matching it exactly matters — someone hunting for a button that is worded differently is precisely the person this table exists to help.
• macOS permissions. «Спеціальні можливості» and «Моніторинг введення» look right, but the same caveat applies as with the Spanish file: only someone reading those panes in Ukrainian can confirm.

Neither blocks anything. The file is a clear net gain as it stands, and both are one-line edits if they need adjusting.

One note for the future, since you now maintain two of these: docs/RELEASING.md step 2 lists the translated READMEs among the docs that block a release until they are back in step. They carry their own copy of the install table, so a renamed installer or a changed permission step makes them wrong. There are no version numbers in them by design, which keeps that bounded to structural changes rather than every release.