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

internal/recipe/context.go

@@ -7,10 +7,11 @@ import (
 )
 
 // ExecContext holds mutable state that persists across recipe steps.
-// It is initialized empty and updated by ENV and WORKDIR steps.
+// It is initialized empty and updated by ENV, WORKDIR, and USER steps.
 type ExecContext struct {
 	WorkDir string
 	EnvVars map[string]string
+	User    string // Current unix user for command execution.
 }
 
 // This regex matches:
@@ -25,7 +26,20 @@ var envRegex = regexp.MustCompile(`\$\$|\$\{([a-zA-Z0-9_]*)\}|\$([a-zA-Z0-9_]+)`
 // If WORKDIR and/or ENV are set, they are prepended as a shell preamble:
 //
 //	cd '/the/dir' && KEY='val' /bin/sh -c 'original command'
+//
+// If USER is set to a non-root user, the entire command is wrapped with su:
+//
+//	su <user> -s /bin/sh -c '<preamble + command>'
 func (c *ExecContext) WrappedCommand(cmd string) string {
+	inner := c.innerCommand(cmd)
+	if c.User != "" && c.User != "root" {
+		return "su " + shellescape(c.User) + " -s /bin/sh -c " + shellescape(inner)
+	}
+	return inner
+}
+
+// innerCommand builds the command with workdir/env preamble but without user wrapping.
+func (c *ExecContext) innerCommand(cmd string) string {
 	prefix := c.shellPrefix()
 	if prefix == "" {
 		return cmd
@@ -42,7 +56,11 @@ func (c *ExecContext) WrappedCommand(cmd string) string {
 // simultaneously before a healthcheck is evaluated.
 func (c *ExecContext) StartCommand(cmd string) string {
 	prefix := c.shellPrefix()
-	return prefix + "nohup /bin/sh -c " + shellescape(cmd) + " >/dev/null 2>&1 &"
+	inner := prefix + "nohup /bin/sh -c " + shellescape(cmd) + " >/dev/null 2>&1 &"
+	if c.User != "" && c.User != "root" {
+		return "su " + shellescape(c.User) + " -s /bin/sh -c " + shellescape(inner)
+	}
+	return inner
 }
 
 // shellPrefix builds the "cd ... && KEY=val " preamble for a shell command.