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

proto/hostagent/hostagent.proto

@@ -119,6 +119,12 @@ message CreateSandboxRequest {
 
   // Template UUID (hex string). Both zeros + team zeros = "minimal" sentinel.
   string template_id = 8;
+
+  // Default unix user for the sandbox (set in envd via PostInit).
+  string default_user = 9;
+
+  // Default environment variables (set in envd via PostInit).
+  map<string, string> default_env = 10;
 }
 
 message CreateSandboxResponse {
@@ -145,6 +151,12 @@ message ResumeSandboxRequest {
   // TTL in seconds restored from the DB so the reaper can auto-pause
   // the sandbox again after inactivity. 0 means no auto-pause.
   int32 timeout_sec = 2;
+
+  // Default unix user for the sandbox (set in envd via PostInit on resume).
+  string default_user = 3;
+
+  // Default environment variables (set in envd via PostInit on resume).
+  map<string, string> default_env = 4;
 }
 
 message ResumeSandboxResponse {