Move web UI from /ui/ to /web/ path prefix

The web UI is now mounted at /web/ (previously /ui/) — cmd/server/main.go
wraps web.Server.Handler with http.StripPrefix("/web", ...), so
internal/web's own routes stay unprefixed (/, /login, /logout,
/shares/..., /static/...) and only the outer mux adds the prefix. All
templates, redirects, and cookie paths updated accordingly. The root '/'
route reverts to the original unauthenticated welcome page (linking to
/web/), and /cal/, /card/, /files/ are unaffected.

Also gitignore the bin/ build output directory.

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

.github/copilot-instructions.md

@@ -4,7 +4,7 @@ A self-hosted CalDAV, CardDAV, and WebDAV server written in Go, backed by a
 filesystem store, with calendar/address book sharing grants tracked in a
 small SQLite database. HTTP Basic Auth (bcrypt) with per-user isolated
 collections. A small server-rendered web UI (templ + Tailwind + htmx) at
-`/ui/` lets users log in and manage their shares.
+`/web/` lets users log in and manage their shares.
 
 ## Build, test, lint
 
@@ -120,10 +120,13 @@ Test files: `internal/store/store_test.go`, `internal/webdav/handler_test.go`,
   the caldav/carddav backends query the shares tables on every request
   (no caching), changes take effect immediately without restarting the
   server.
-- `internal/web` — the web UI, mounted at `/ui/` in `cmd/server/main.go`
-  (`web.NewServer(cfg, st, dbase, logger).Handler(webstatic.FS())`),
-  entirely separate from `internal/auth`'s Basic Auth: logins go through
-  `/ui/login` (username/password checked against `cfg.Users` the same way
+- `internal/web` — the web UI, mounted at `/web/` in `cmd/server/main.go`
+  (`mux.Handle("/web/", http.StripPrefix("/web", web.NewServer(cfg, st,
+  dbase, logger).Handler(webstatic.FS())))`, so `Server.Handler`'s own
+  routes are all unprefixed — `/login`, `/`, `/shares/...` — and only the
+  outer mux adds the `/web` prefix), entirely separate from `internal/auth`'s
+  Basic Auth: logins go through
+  `/web/login` (username/password checked against `cfg.Users` the same way
   Basic Auth does, via bcrypt) and issue an opaque random session token
   stored in the `web_sessions` SQLite table (`db.CreateSession`/
   `SessionUser`/`DeleteSession`, see `internal/db/sessions.go`), set as an
@@ -135,7 +138,7 @@ Test files: `internal/store/store_test.go`, `internal/webdav/handler_test.go`,
   (`SharesOfCalendar`/`SharesOfAddressBook`) and what's shared with them
   (`CalendarsSharedWith`/`AddressBooksSharedWith`). `internal/web/shares.go`
   handles POST (create/update share) and DELETE (revoke) at
-  `/ui/shares/{calendar,addressbook}`, re-rendering just the affected
+  `/web/shares/{calendar,addressbook}`, re-rendering just the affected
   resource card for htmx's `hx-swap="outerHTML"`; it always checks
   `ownsResource` first so a user can only share resources actually
   configured for their own account (never someone else's, even via a