Add disk_size_mb, auto-expand base images, admin templates endpoint

Disk sizing:
- Add disk_size_mb column to sandboxes table (default 20480 = 20GB)
- Add disk_size_mb to CreateSandboxRequest proto, passed through the
  full chain: service → RPC → host agent → sandbox manager → devicemapper
- devicemapper.CreateSnapshot takes separate cowSizeBytes param so the
  sparse CoW file can be sized independently from the origin
- EnsureImageSizes() runs at host agent startup: expands any base image
  smaller than 20GB via truncate + resize2fs (sparse, no extra physical
  disk). Sandboxes then get the full 20GB via fast dm-snapshot path
- FlattenRootfs shrinks output images with resize2fs -M so stored
  templates are compact; EnsureImageSizes re-expands on next startup

Admin templates visibility:
- Add GET /v1/admin/templates endpoint listing all templates across teams
- Frontend admin templates page uses listAdminTemplates() instead of
  team-scoped listSnapshots()
- Platform templates (team_id = all-zeros UUID) now visible to all teams:
  GetTemplateByTeam, ListTemplatesByTeam, ListTemplatesByTeamAndType
  queries include platform team_id in WHERE clause

frontend/src/lib/api/builds.ts

@@ -50,3 +50,17 @@ export async function listBuilds(): Promise<ApiResult<Build[]>> {
 export async function getBuild(id: string): Promise<ApiResult<Build>> {
 	return apiFetch('GET', `/api/v1/admin/builds/${id}`);
 }
+
+export type AdminTemplate = {
+	name: string;
+	type: string;
+	vcpus: number;
+	memory_mb: number;
+	size_bytes: number;
+	team_id: string;
+	created_at: string;
+};
+
+export async function listAdminTemplates(): Promise<ApiResult<AdminTemplate[]>> {
+	return apiFetch('GET', '/api/v1/admin/templates');
+}