[0.4.x] Plugins: Reconcile and complete the German translation

[fmolara]

The German plugin catalog leaves the debug input's help text in English and
misspells the Bumpscope plugin name in its translated description. Complete
the missing help text and correct that name; the catalog now has 109
translated messages and no fuzzy or untranslated entries.

This follows reconciliation of Chris Leick's 2023 submission in
Debian #1033190, which targeted 0.4.0. The
current 0.4.2 catalog already contains the translations that still match
that submission, as well as newer PortAudio/debug entries and %zu format
adjustments. Preserve those existing translations instead of replacing the
catalog with the older attachment.

Retain Chris's translator credit, including the 2023 revision recorded in
his submission. The new debug help translation and Bumpscope correction are
maintainer additions; record Federico as the last translator and update the
project version, revision date and German plural rule.

Only libvisual-plugins/po/de.po changes. This is independent of the core
translation PR #427 and targets 0.4.x.

Validation:

• msgfmt --check --check-compatibility --statistics: 109 translated messages;
• msgcmp against libvisual-plugins-0.4.pot: complete coverage;
• compiled-catalog comparison: 107 existing translations unchanged, one
  proper-name correction and one newly translated description;
• the equivalent Debian patch produces the same translations, allowing for
  the English Transparant bars -> Transparent bars rename already in
  upstream after 0.4.2;
• the same completeness check rejects the unmodified catalog.