Add HTMX-based UI templates and handlers for account and audit management

- Introduced `web/templates/` for HTMX-fragmented pages (`dashboard`, `accounts`, `account_detail`, `error_fragment`, etc.).
- Implemented UI routes for account CRUD, audit log display, and login/logout with CSRF protection.
- Added `internal/ui/` package for handlers, CSRF manager, session validation, and token issuance.
- Updated documentation to include new UI features and templates directory structure.
- Security: Double-submit CSRF cookies, constant-time HMAC validation, login password/Argon2id re-verification at all steps to prevent bypass.

internal/ui/csrf.go

@@ -0,0 +1,65 @@
+// Package ui provides the HTMX-based management web interface for MCIAS.
+package ui
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/hex"
+	"fmt"
+)
+
+// CSRFManager implements HMAC-signed Double-Submit Cookie CSRF protection.
+//
+// Security design:
+//   - The CSRF key is derived from the server master key via SHA-256 with a
+//     domain-separation prefix, so it is unique to the UI CSRF function.
+//   - The cookie value is 32 bytes of cryptographic random (non-HttpOnly so
+//     HTMX can read it via JavaScript-free double-submit; SameSite=Strict
+//     provides the primary CSRF defence for browser-initiated requests).
+//   - The form/header value is HMAC-SHA256(key, cookieVal); this is what the
+//     server verifies. An attacker cannot forge the HMAC without the key.
+//   - Comparison uses crypto/subtle.ConstantTimeCompare to prevent timing attacks.
+type CSRFManager struct {
+	key []byte
+}
+
+// newCSRFManager creates a CSRFManager whose key is derived from masterKey.
+// Key derivation: SHA-256("mcias-ui-csrf-v1" || masterKey)
+func newCSRFManager(masterKey []byte) *CSRFManager {
+	h := sha256.New()
+	h.Write([]byte("mcias-ui-csrf-v1"))
+	h.Write(masterKey)
+	return &CSRFManager{key: h.Sum(nil)}
+}
+
+// NewToken generates a fresh CSRF token pair.
+//
+// Returns:
+//   - cookieVal: hex(32 random bytes) — stored in the mcias_csrf cookie
+//   - headerVal: hex(HMAC-SHA256(key, cookieVal)) — embedded in forms / X-CSRF-Token header
+func (c *CSRFManager) NewToken() (cookieVal, headerVal string, err error) {
+	raw := make([]byte, 32)
+	if _, err = rand.Read(raw); err != nil {
+		return "", "", fmt.Errorf("csrf: generate random bytes: %w", err)
+	}
+	cookieVal = hex.EncodeToString(raw)
+	mac := hmac.New(sha256.New, c.key)
+	mac.Write([]byte(cookieVal))
+	headerVal = hex.EncodeToString(mac.Sum(nil))
+	return cookieVal, headerVal, nil
+}
+
+// Validate verifies that headerVal is the correct HMAC of cookieVal.
+// Returns false on any mismatch or decoding error.
+func (c *CSRFManager) Validate(cookieVal, headerVal string) bool {
+	if cookieVal == "" || headerVal == "" {
+		return false
+	}
+	mac := hmac.New(sha256.New, c.key)
+	mac.Write([]byte(cookieVal))
+	expected := hex.EncodeToString(mac.Sum(nil))
+	// Security: constant-time comparison prevents timing oracle attacks.
+	return subtle.ConstantTimeCompare([]byte(expected), []byte(headerVal)) == 1
+}