Add web UI: login, dashboard, and share management (templ + Tailwind + htmx)

New internal/web package mounted at /ui/, separate from DAV Basic Auth:

- Cookie-based sessions (opaque random tokens in a new web_sessions
  SQLite table, internal/db/sessions.go), checked against the same
  cfg.Users/bcrypt credentials as DAV Basic Auth.
- Dashboard listing the logged-in user's own calendars/address books,
  who they're shared with, and what's shared with them.
- Share/unshare directly from the dashboard, updated in place via htmx
  partial swaps (POST to create/update, DELETE to revoke). Always
  verifies the resource actually belongs to the logged-in user before
  granting a share.
- Templates written in templ (internal/web/templates/*.templ, generated
  *_templ.go committed), styled with Tailwind CSS v4 (web/input.css,
  compiled to web/static/app.css), with htmx vendored as a static file
  for the dynamic bits. Both are embedded into the binary at build time
  (web/staticassets.go) so the compiled server has no Node.js/web/
  runtime dependency.
- Wired into cmd/server/main.go at /ui/ alongside the existing /cal/,
  /card/, /files/ routes; welcome page links to it.
- Tests: internal/web/server_test.go covers login success/failure, the
  login-required redirect, dashboard rendering, share/unshare including
  the htmx-v2-sends-DELETE-params-as-query-string quirk, and rejecting
  shares of resources the user doesn't own.
- Docs: README (new 'Web UI' section, updated sharing section, project
  layout, dependencies) and copilot-instructions updated accordingly.
  Makefile: new templ-generate/web-deps/web-css targets.

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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build run test tidy lint docker hash-password nidusctl
+.PHONY: build run test tidy lint docker hash-password nidusctl web-deps web-css templ-generate
 
 ## build: compile the binary
 build:
@@ -38,3 +38,16 @@ hash-password:
 ## Usage: make nidusctl ARGS="calendar share alice work bob write"
 nidusctl: build
 	./bin/nidusctl -config config.yaml $(ARGS)
+
+## templ-generate: regenerate *_templ.go files from internal/web/templates/*.templ
+templ-generate:
+	templ generate
+
+## web-deps: install the Tailwind CLI (needed once, or after web/package.json changes)
+web-deps:
+	cd web && npm install
+
+## web-css: rebuild the compiled/embedded Tailwind CSS (web/static/app.css)
+## Run this after editing any .templ file or web/input.css.
+web-css: templ-generate
+	cd web && npx @tailwindcss/cli -i ./input.css -o ./static/app.css --minify