Add per-channel hybrid audio spread spectrum and env feature toggles

Spread spectrum v2: independent per-channel embedding with round-robin
bit distribution, preserving spatial stereo/surround mix. Adaptive chip
tiers (256/512/1024) trade capacity for lossy codec robustness. LFE
channel skipped for 5.1+ layouts. v2 header (20B) with backward-
compatible v0 decode fallback.

Environment toggles (STEGASOO_AUDIO, STEGASOO_VIDEO) gate audio/video
features for minimal builds (e.g. Raspberry Pi image-only). Values:
auto (default, detect deps), 1/true (force on), 0/false (force off).

Web UI fixes: accordion defaults to step 1 on load, chevron arrow
styling, required attribute toggling for audio carrier type switch,
"Images & Mode" renamed to "Reference, Carrier, Mode".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontends/web/ssl_utils.py

@@ -81,10 +81,12 @@ def generate_self_signed_cert(
     )
 
     # Create certificate
-    subject = issuer = x509.Name([
-        x509.NameAttribute(NameOID.ORGANIZATION_NAME, "Stegasoo"),
-        x509.NameAttribute(NameOID.COMMON_NAME, hostname),
-    ])
+    subject = issuer = x509.Name(
+        [
+            x509.NameAttribute(NameOID.ORGANIZATION_NAME, "Stegasoo"),
+            x509.NameAttribute(NameOID.COMMON_NAME, hostname),
+        ]
+    )
 
     # Subject Alternative Names
     san_list = [
@@ -112,7 +114,7 @@ def generate_self_signed_cert(
         except (ipaddress.AddressValueError, ValueError):
             pass
 
-    now = datetime.datetime.now(datetime.timezone.utc)
+    now = datetime.datetime.now(datetime.UTC)
     cert = (
         x509.CertificateBuilder()
         .subject_name(subject)