feat(auth): login, roles, user mgmt, setup wizard, and session guard

- New skald-setup crate: interactive first-run wizard that creates the
  admin user, prompts for encryption choice and password
- Auth system: session-based login/logout with cookie, guard middleware
- Roles API: CRUD for data-driven roles, seeded on first boot
- Users management API: create, list, edit, delete users
- Setup state API: check if first admin has been created
- Frontend: login-page, setup-page, users-page, roles-page, profile-page
  components with corresponding CSS
- Topbar: avatar dropdown with profile link and logout
- Sidebar: nav entries for Users and Roles (admin only)
- Page shell CSS: layout support for the new pages
- build.sh: builds both skald and skald-setup binaries
- run.sh: runs skald-setup before the server loop
- CLAUDE.md: updated workspace layout and build/run docs

crates/skald-core/src/skald/accessors.rs

@@ -52,6 +52,7 @@ impl Skald {
     // Runtime / cross-cutting
     pub fn db(&self) -> &Arc<SqlitePool> { &self.rt.db }
     pub fn users(&self) -> &Arc<UserManager> { &self.rt.users }
+    pub fn sessions(&self) -> &Arc<crate::auth::SessionStore> { &self.rt.sessions }
     pub fn config(&self) -> &Arc<GlobalConfigManager> { &self.rt.config }
     pub fn config_properties(&self) -> &[core_api::ConfigSet] { &self.rt.config_properties }
     pub fn system_bus(&self) -> &Arc<SystemEventBus> { &self.rt.system_bus }