feat: wire auth into the web layer (Schritt 2, part 2/2)

Registration creates a new account plus its first user; login
authenticates an existing one; both set a deklarix_session cookie
(HttpOnly, SameSite=Strict, Secure only when the request itself came
over TLS — hardcoding Secure=true would break local http://localhost
development, since browsers won't store a Secure cookie over plaintext).

requirePage protects full-page GETs (redirects to /login); requireAPI
protects the htmx/download endpoints (401, since those are only ever
called from an already-authenticated page — an unauthenticated hit
there is the exception, e.g. a session expiring mid-use).

handleCheck now creates submissions under the current account.
handleArchive and handleDossierDownload compare the submission's
account against the caller's and return 404 on mismatch — not 403,
which would confirm the ID exists to a different tenant. Login failure
uses the same message for "no such email" and "wrong password" to avoid
account enumeration.

Restructured templates along the way: layout.html now only holds
reusable fragments ("head", "nav"); each full page (index/login/register)
is its own top-level named template. The previous layout+content nesting
would have broken the moment a second page defined "content" — Go's
html/template keys blocks by name across the whole parsed set, not per
file, so two pages both defining "content" would silently overwrite each
other.

Verified against a real running instance (not just Go's test recorder):
started the compiled binary against a fresh Postgres and drove the whole
flow with curl — anonymous redirect, registration setting a real cookie,
authenticated page load, logout clearing both the cookie and the
server-side session row, and being locked out again afterward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/go-pdf/fpdf v0.9.0
 	github.com/golang-migrate/migrate/v4 v4.19.1
 	github.com/jackc/pgx/v5 v5.10.0
+	golang.org/x/crypto v0.55.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
@@ -18,7 +19,6 @@ require (
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/kr/text v0.2.0 // indirect
 	github.com/rogpeppe/go-internal v1.16.0 // indirect
-	golang.org/x/crypto v0.55.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/text v0.41.0 // indirect
 )