[Feature]: Add Polish (pl) UI locale, native speaker review

[levy52]

What problem are you trying to solve?

Native speaker: yes — Polish is my first language, and this is the only translation set
I'll be responsible for end to end.

OpenKnowledge has no Polish (pl) UI locale. I read Polish natively and want to use the
app interface (web and desktop) in Polish, then keep reviewing wording in-product as I go.

Proposed solution

Add pl (language tag, not pl-PL) as a picker locale and ship a complete catalog in one PR.

• Tag: pl. LTR direction. No deferred layout locale.
• Plural: CLDR one/few/many/other (four categories, not just other — this differs
  from ko). Plural-Forms header set accordingly so check-i18n-picker-completeness.mjs
  and the gettext tooling see correct pluralization from the first commit.
• Scope: app interface (web + desktop) + native desktop menus via the same messages.json.
  Not the docs site, not the CLI.
• REVIEW.md: will set status explicitly in the PR rather than leaving it implied — status:
  unreviewed until I say otherwise, this PR is the first read.

Locked glossary I'll use:

English	Polish
document	dokument
folder	folder
workspace	obszar roboczy
knowledge base	baza wiedzy
wiki-link	wiki link
agent	agent
checkpoint	punkt kontrolny
branch	branch

I'll open one complete-catalog PR once no msgstr values are empty. I won't open an empty
or partial catalog PR.

Area

Editor

Alternatives considered

No response