Add admin capsule management, fix file browser for special files, normalize dialog styles

- Admin capsule CRUD: list, create (platform templates), get detail with
  terminal/files/metrics, snapshot, destroy
- First signup auto-promotes to platform admin
- JWT auth via query param for WebSocket connections
- File browser: handle non-regular files (devices, pipes, sockets) gracefully
  instead of showing raw backend errors
- Normalize admin template dialogs to match established dialog patterns:
  remove accent bars, unify animation/shadow/button styles

db/migrations/20260412213141_seed_platform_team.sql

@@ -0,0 +1,12 @@
+-- +goose Up
+
+-- Seed the platform team row. This is the sentinel team (all-zeros UUID) that
+-- owns platform-wide resources: global templates, admin-created capsules, etc.
+-- No user can become a member of this team — it exists solely to satisfy
+-- foreign key constraints and to act as a namespace for platform resources.
+INSERT INTO teams (id, name, slug)
+VALUES ('00000000-0000-0000-0000-000000000000', 'Platform', 'platform')
+ON CONFLICT (id) DO NOTHING;
+
+-- +goose Down
+DELETE FROM teams WHERE id = '00000000-0000-0000-0000-000000000000';