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/auth.py

@@ -77,14 +77,10 @@ def init_db():
     db = get_db()
 
     # Check if we need to migrate from old single-user schema
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='admin_user'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='admin_user'")
     has_old_table = cursor.fetchone() is not None
 
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='users'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='users'")
     has_new_table = cursor.fetchone() is not None
 
     if has_old_table and not has_new_table:
@@ -189,9 +185,7 @@ def _ensure_channel_keys_table(db: sqlite3.Connection):
 
 def _ensure_app_settings_table(db: sqlite3.Connection):
     """Ensure app_settings table exists (v4.1.0 migration)."""
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='app_settings'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='app_settings'")
     if cursor.fetchone() is None:
         db.executescript("""
             CREATE TABLE IF NOT EXISTS app_settings (
@@ -212,9 +206,7 @@ def _ensure_app_settings_table(db: sqlite3.Connection):
 def get_app_setting(key: str) -> str | None:
     """Get an app-level setting value."""
     db = get_db()
-    row = db.execute(
-        "SELECT value FROM app_settings WHERE key = ?", (key,)
-    ).fetchone()
+    row = db.execute("SELECT value FROM app_settings WHERE key = ?", (key,)).fetchone()
     return row["value"] if row else None
 
 
@@ -384,12 +376,10 @@ def get_user_by_username(username: str) -> User | None:
 def get_all_users() -> list[User]:
     """Get all users, admins first, then by creation date."""
     db = get_db()
-    rows = db.execute(
-        """
+    rows = db.execute("""
         SELECT id, username, role, created_at FROM users
         ORDER BY role = 'admin' DESC, created_at ASC
-        """
-    ).fetchall()
+        """).fetchall()
     return [
         User(
             id=row["id"],
@@ -596,9 +586,7 @@ def create_admin_user(username: str, password: str) -> tuple[bool, str]:
     return success, msg
 
 
-def change_password(
-    user_id: int, current_password: str, new_password: str
-) -> tuple[bool, str]:
+def change_password(user_id: int, current_password: str, new_password: str) -> tuple[bool, str]:
     """Change a user's password (requires current password)."""
     user = get_user_by_id(user_id)
     if not user:
@@ -667,9 +655,7 @@ def delete_user(user_id: int, current_user_id: int) -> tuple[bool, str]:
     # Check if this is the last admin
     if user.role == ROLE_ADMIN:
         db = get_db()
-        admin_count = db.execute(
-            "SELECT COUNT(*) FROM users WHERE role = 'admin'"
-        ).fetchone()[0]
+        admin_count = db.execute("SELECT COUNT(*) FROM users WHERE role = 'admin'").fetchone()[0]
         if admin_count <= 1:
             return False, "Cannot delete the last admin"
 
@@ -848,9 +834,7 @@ def save_channel_key(
         return False, "This channel key is already saved", None
 
 
-def update_channel_key_name(
-    key_id: int, user_id: int, new_name: str
-) -> tuple[bool, str]:
+def update_channel_key_name(key_id: int, user_id: int, new_name: str) -> tuple[bool, str]:
     """Update the name of a saved channel key."""
     new_name = new_name.strip()
     if not new_name: