chore(plugins): fix notes left stale by the plugin manager cleanup

[datlechin]

Why

#3482 left two comments describing code it deleted, and shipped a visible change without a CHANGELOG line.

Change

• ColumnDefaultVocabulary no longer points at the deleted PluginMetadataRegistry.fallbackCategory.
• The install progress panel's comment matches the installer: no fixed 0 first, and a download without a Content-Length reports only the final 1.
• CHANGELOG, Changed: plugin download progress follows the bytes instead of jumping to full at the end.

Verified

Comments and CHANGELOG only. ColumnDefaultVocabularyTests and the app build pass.

Not in this PR

• No shared isCompatibleKit helper. validateBundleVersions is the one gate before atomicReplace and needs the direction for its two errors; rejectedAction and resolvedBinary take the bounds as parameters for their tests.