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/api/auth.py

@@ -17,9 +17,8 @@ import json
 import os
 import secrets
 from pathlib import Path
-from typing import Optional
 
-from fastapi import Depends, HTTPException, Security
+from fastapi import HTTPException, Security
 from fastapi.security import APIKeyHeader
 
 # API key header name
@@ -55,7 +54,7 @@ def _load_keys(location: str = "user") -> dict:
         try:
             with open(keys_file) as f:
                 return json.load(f)
-        except (json.JSONDecodeError, IOError):
+        except (OSError, json.JSONDecodeError):
             return {"keys": [], "enabled": True}
     return {"keys": [], "enabled": True}
 
@@ -101,11 +100,13 @@ def add_api_key(name: str, location: str = "user") -> str:
         if existing["name"] == name:
             raise ValueError(f"Key with name '{name}' already exists")
 
-    data["keys"].append({
-        "name": name,
-        "hash": key_hash,
-        "created": __import__("datetime").datetime.now().isoformat(),
-    })
+    data["keys"].append(
+        {
+            "name": name,
+            "hash": key_hash,
+            "created": __import__("datetime").datetime.now().isoformat(),
+        }
+    )
 
     _save_keys(data, location)
 
@@ -204,12 +205,12 @@ def get_api_key_status() -> dict:
         "keys": {
             "user": user_keys,
             "project": project_keys,
-        }
+        },
     }
 
 
 # FastAPI dependency for API key authentication
-async def require_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) -> str:
+async def require_api_key(api_key: str | None = Security(API_KEY_HEADER)) -> str:
     """
     FastAPI dependency that requires a valid API key.
 
@@ -243,7 +244,7 @@ async def require_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) ->
     return api_key
 
 
-async def optional_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) -> Optional[str]:
+async def optional_api_key(api_key: str | None = Security(API_KEY_HEADER)) -> str | None:
     """
     FastAPI dependency that optionally validates API key.