fix(gallery): allow uninstalling orphaned meta backends + force reinstall (#9434)

Two interrelated bugs that combined to make a meta backend impossible
to uninstall once its concrete had been removed from disk (partial
install, earlier crash, manual cleanup).

1. DeleteBackendFromSystem returned "meta backend %q not found" and
   bailed out early when the concrete directory didn't exist,
   preventing the orphaned meta dir from ever being removed. Treat a
   missing concrete as idempotent success — log a warning and continue
   to remove the orphan meta.

2. InstallBackendFromGallery's "already installed, skip" short-circuit
   only checked that the name was known (`backends.Exists(name)`); an
   orphaned meta whose RunFile points at a missing concrete still
   satisfies that check, so every reinstall returned nil without doing
   anything. Afterwards the worker's findBackend returned empty and we
   kept looping with "backend %q not found after install attempt".
   Require the entry to be actually runnable (run.sh stat-able, not a
   directory) before skipping.

New helper isBackendRunnable centralises the runnability test so both
the install guard and future callers stay in sync. Tests cover the
orphaned-meta delete path and the non-runnable short-circuit case.