* feat(providers): bundle metadata for the 0.63.0 registry

Add the Pi provider official CodexBar 0.63.0 adds to the registry:
bundled fallback name, an original pi-glyph icon, and the verified
docs/pi.md path. Brand color and dashboard/login/status links are
omitted pending verifiable upstream sources; the widget uses the
theme highlight and hides missing links by design.

The provider icon check now requires pi.svg, the QtTest pins the
minimal bundled metadata (verified by breaking the docs path and
confirming the test goes red), and the translation catalogs gain
the unchanged Pi name. README and the usage guide describe the 75
provider registry, and the 0.63.0 parity review is recorded and
indexed. No new static source assertions: only the existing
provider-count strings move 74->75 alongside the docs.

* fix(i18n): keep the Pi brand name unchanged in every catalog

msgcmp without --use-untranslated ignores untranslated entries when
covering the template, so the empty Pi msgstr failed
check-python-test_localized_cli_labels and the smoke-runtime catalog
compilation with 'not translated / not used'. Brand names stay
unchanged per docs/translations.md, matching the 0.61.0 precedent
(msgstr "Muse Code"), so every catalog now carries msgstr "Pi".

Also takes the CodeRabbit grammar nit on the CHANGELOG entry
(provider that official CodexBar 0.63.0 adds).