Move users, calendars, and address books from config.yaml into the database

BREAKING CHANGE: the users:/config-based collection setup is gone. All
user, calendar, and address-book data now lives in the SQLite DB
(internal/db) and is managed exclusively via nidusctl or the web UI.
Existing deployments must recreate their users after upgrading:
  nidusctl user create <username>
  nidusctl calendar create <username> <name>
  nidusctl addressbook create <username> <name>

- internal/db: new users, calendars, addressbooks tables with FK cascade
  delete; foreign_keys pragma enabled; internal/db/users.go implements
  full CRUD + bcrypt auth (CreateUser, VerifyPassword, ListUsers,
  CreateCalendar/AddressBook, etc).
- internal/config: removed Users/UserConfig entirely.
- internal/auth: Basic Auth now checks credentials via db.DB instead of
  cfg.Users.
- internal/caldav, internal/carddav: ListCalendars/ListAddressBooks and
  Create/Delete now backed by the DB.
- internal/web: login uses db.VerifyPassword; new resources.go adds
  create/delete handlers for calendars/address books at
  /web/resources/{calendar,addressbook}; dashboard gained create forms
  and per-card delete buttons (templ + htmx, no hyperscript).
- tools/nidusctl: new user create/delete/list/passwd commands (masked
  interactive password prompt via golang.org/x/term) plus create/delete/
  list subcommands for calendar/addressbook.
- cmd/server/main.go: pre-creates on-disk collections from the DB at
  startup instead of cfg.Users; warns when no users exist yet.
- Updated tests to seed data via the DB; added resources_test.go for the
  new web UI handlers.
- README.md and .github/copilot-instructions.md updated to document the
  new nidusctl commands and the DB-backed architecture.

Verified end-to-end against a live test server: nidusctl user/calendar/
addressbook create, DAV Basic Auth PROPFIND, web login, dashboard
rendering, and web UI create/delete of resources all confirmed working.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/web/shares.go

@@ -24,11 +24,6 @@ func (s *Server) handleAddressBookShare(w http.ResponseWriter, r *http.Request)
 func (s *Server) handleShare(w http.ResponseWriter, r *http.Request, kind string) {
 	username := userFromContext(r.Context())
 
-	if s.dbase == nil {
-		http.Error(w, "sharing is not available (no database configured)", http.StatusServiceUnavailable)
-		return
-	}
-
 	// htmx v2 sends DELETE request parameters (including hx-vals) as URL
 	// query parameters, not a request body — unlike POST/PUT/PATCH.
 	if r.Method == http.MethodDelete {
@@ -100,15 +95,18 @@ func (s *Server) handleShare(w http.ResponseWriter, r *http.Request, kind string
 // actually configured for username, to prevent sharing arbitrary/other
 // users' resources via a forged form post.
 func (s *Server) ownsResource(username, kind, resource string) bool {
-	user, ok := s.cfg.Users[username]
-	if !ok {
-		return false
-	}
-	var list []string
+	var (
+		list []string
+		err  error
+	)
 	if kind == "calendar" {
-		list = user.Calendars
+		list, err = s.dbase.ListCalendars(username)
 	} else {
-		list = user.AddressBooks
+		list, err = s.dbase.ListAddressBooks(username)
+	}
+	if err != nil {
+		s.logger.Warn("checking resource ownership", "kind", kind, "error", err)
+		return false
 	}
 	for _, n := range list {
 		if n == resource {