Add USER, COPY, ENV persistence to template build system

Implement three new recipe commands for the admin template builder:

- USER <name>: creates the user (adduser + passwordless sudo), switches
  execution context so subsequent RUN/START commands run as that user
  via su wrapping. Last USER becomes the template's default_user.

- COPY <src> <dst>: copies files from an uploaded build archive
  (tar/tar.gz/zip) into the sandbox. Source paths validated against
  traversal. Ownership set to the current USER.

- ENV persistence: accumulated env vars stored in templates.default_env
  (JSONB) and injected via PostInit when sandboxes are created from the
  template, mirroring Docker's image metadata approach.

Supporting changes:
- Pre-build creates wrenn-user as default (via USER command)
- WORKDIR now creates the directory if it doesn't exist (mkdir -p)
- Per-step progress updates (ProgressFunc callback) for live UI
- Multipart form support on POST /v1/admin/builds for archive upload
- Proto: default_user/default_env fields on Create/ResumeSandboxRequest
- Host agent: SetDefaults calls PostInitWithDefaults on envd
- Control plane: reads template defaults, passes on sandbox create/resume
- Frontend: file upload widget, recipe copy button, keyword colors for
  USER/COPY, fixed Svelte whitespace stripping in step display
- Admin panel defaults to /admin/templates instead of /admin/hosts
- Migration adds default_user and default_env to templates and
  template_builds tables

db/migrations/20260411182550_template_defaults.sql

@@ -0,0 +1,17 @@
+-- +goose Up
+ALTER TABLE templates
+    ADD COLUMN default_user TEXT NOT NULL DEFAULT 'root',
+    ADD COLUMN default_env  JSONB NOT NULL DEFAULT '{}';
+
+ALTER TABLE template_builds
+    ADD COLUMN default_user TEXT NOT NULL DEFAULT 'root',
+    ADD COLUMN default_env  JSONB NOT NULL DEFAULT '{}';
+
+-- +goose Down
+ALTER TABLE template_builds
+    DROP COLUMN default_env,
+    DROP COLUMN default_user;
+
+ALTER TABLE templates
+    DROP COLUMN default_env,
+    DROP COLUMN default_user;