fix: show virtual systems in the Library system list

[wizzomafizzo]

The Library tab's top-level system list should include virtual systems (Core
launchables), the way zaparoo-frontend does. Right now it drops them.

Excluding them from the other system lists is correct — search pickers,
create.search.tsx, and the SystemSelector instances all pick a system to
search or browse within, and a virtual system has no media inside it. This is
only about the top-level list you land on in the Library tab, where each entry is
a thing you can pick and write to a token.

Core returns virtual systems from systems with zapScript set to their
zaparoo://<uuid>/<name> URI and mediaCount: 0. The Library list filters
through filterSystemCatalog (src/routes/library.index.tsx:105), which applies
systemHasIndexedMedia (src/lib/systemFilters.ts:23-27):

return system.mediaCount === undefined || system.mediaCount > 0;

zaparoo-frontend uses the zap script as the signal instead of treating a zero
count as "empty" — rust/zaparoo-core/src/endpoints/catalog.rs:63:

!system.zap_script.trim().is_empty() || system.media_count.is_some_and(|count| count > 0)

Two notes on implementing it:

• The existing includeEmptySystems option is too blunt here. It would also
  admit real systems with zero indexed media, which library.index.test.tsx:145
  ("should hide systems with an explicit zero media count") deliberately keeps
  hidden. The frontend's rule — keep when zapScript is non-empty or media
  count > 0 — draws the line in the right place.
• The app never reads System.zapScript anywhere today, so listing a virtual
  system also needs a way to write it to a token, otherwise the entry is inert.
  availableSystemCount (library.index.tsx:128) gates the empty state and
  would need the same treatment.

This came up with a Windows user whose custom Winamp launcher appeared to do
nothing. Platforms without an on-device frontend feel it most: Core serves
/app/ from the embedded zaparoo-app build, so on Windows the app and the web
UI are the same code and a kind = "virtual_system" custom launcher is not
visible anywhere.