feat(skills): add Import from local to the New skill dialog (MUL-6703) (#7570)

Adds an "Import from local" route to the New skill dialog, so a member can pick
a folder containing SKILL.md or a .skill / .zip archive and import it straight
into the workspace. Shows name, description and file count before importing,
and fails clearly on a missing SKILL.md, oversized input or a name conflict
instead of creating a partial skill. Reuses the existing POST /api/skills/import
endpoint; web and desktop share the entry point. Docs and locales updated for
all four languages.

Directory selection is decided from path + size before any file is read, so a
large project folder cannot be pulled into memory. Concurrent selections are
guarded by a generation counter, and the import response is parsed through a
zod schema before its status is branched on.

Co-authored-by: J <bohan@devv.ai>