[web] Migrate manual plurals to i18next _plural convention (#33989)

* Add tests on plurals

* Update `collabs_per_proj` and its pluralisations

* Update `n_user` and its pluralisations

* Update `showing_x_results` and its pluralisations

* Update `show_x_more_projects` and its pluralisations

* `bin/run web npm run extract-translations`

* Populate `_plural` keys in non-en locales

For 2-form languages (da, de, es, fi, fr, it, nl, no, pt, sv, tr), copy
the existing bare-key value into the new `_plural` sibling to prevent
i18next from falling back to English for count!=1.

Also remove orphan singular keys (`collabs_per_proj_single`,
`showing_1_result*`) left over from the previous commits.

Bare-key values remain in their original plural form pending translator
review — count=1 will still render the plural form in non-en until
translators flip those to singular. Multi-form (cs, pl, ru) and
single-form (ja, ko, zh-CN, zh-TW) locales are unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* Flip non-en bare-key values to singular form

Per review, the i18next v3 plural convention uses the bare key for count=1
(singular) and `_plural` for count!=1. The non-en bare-key values were
left as the original plural form by the previous commit so the `_plural`
siblings could be copied from them; this commit flips the bare values to
the singular form per language.

Languages where singular and plural noun forms coincide (Finnish, Swedish,
Turkish) are unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* Apply suggestions from code review

Co-authored-by: Olzhas Askar <olzhas.askar@gmail.com>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Olzhas Askar <olzhas.askar@gmail.com>
GitOrigin-RevId: 628513ca792c2dcce023247e52b7320e2741cc54

services/web/locales/ru.json

@@ -88,7 +88,7 @@
   "cn": "Китайский (упрощённый)",
   "collaboration": "Совместная разработка",
   "collaborator": "Совместная работа",
-  "collabs_per_proj": "Максимальное число соавторов на проект: __collabcount__",
+  "collabs_per_proj": "Максимальное число соавторов на проект: __count__",
   "comment": "Комментарии",
   "common": "Общие",
   "compile_larger_projects": "Компиляция больших проектов",