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>

web/input.css

@@ -0,0 +1,5 @@
+@import "tailwindcss";
+
+/* Scan generated Go templ files (and any hand-written HTML) for class names */
+@source "../internal/web/templates/**/*.templ";
+@source "../internal/web/templates/**/*_templ.go";