files: per-user public at ~name/public; home = member directory

Re-model the web file host as a file server (not a website host):

- Drop the misnamed /site area. A member's public files are now their
  /me/public subfolder (unix ~/public), served anonymously at
  ~<name>/public. The rest of /me stays private; only ~name/public is
  ever exposed. Bare /~name redirects to /~name/public/.
- The root / is now a directory of ALL members, each linked to their BBS
  site (https://<bbs-host>/~name via WebConfig.SiteBase) AND their public
  files here (~name/public). No longer hides empty members.
- Sites/homepages stay on the BBS — files.<host> only links to them.
- Usage gauge is just /me again (which includes /me/public).

setup.sh + docs updated; tests cover ~name/public browse/download, the
bare-~name redirect, empty-member empty-listing, /public-only exposure,
and traversal confinement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/files/e2e_test.go

@@ -41,8 +41,8 @@ func TestE2E_RootListing(t *testing.T) {
 		names = append(names, fi.Name())
 	}
 	sort.Strings(names)
-	if len(names) != 3 || names[0] != "me" || names[1] != "public" || names[2] != "site" {
-		t.Errorf("root listing = %v, want [me public site]", names)
+	if len(names) != 2 || names[0] != "me" || names[1] != "public" {
+		t.Errorf("root listing = %v, want [me public]", names)
 	}
 }