security: fix SAS verification bypass and unauthenticated frame injection; release v5.5.2

A security review of the transport and verification layers. Every item is a fix
to how untrusted peer input is handled; no features changed.

- SAS verification could be bypassed. `verification_both_confirmed` is an
  unauthenticated frame on a channel that is not yet trusted, but it was taken as
  proof that both sides had compared their codes — so a peer who completed the
  signalling exchange could send it right after the data channel opened and drive
  the other side to a "verified" session while the user never looked at the code.
  It is now only an acknowledgement: refused unless this side already confirmed
  locally, and _setVerifiedStatus() independently rejects any SAS-based
  transition without a local confirmation. Holding ECDH-derived keys was never
  proof of identity — a MITM has those too.

- Unauthenticated frames could be injected into the chat. A bare
  {type:"message"} frame, a raw non-JSON frame and a binary frame were each
  decoded and rendered, bypassing decryption, the HMAC check and the verification
  gate; the injected text was indistinguishable from a genuine message. Chat
  content now reaches the UI only through the authenticated enhanced_message
  path.

- A peer could supply the verification code. `sas_code` announcements were
  adopted verbatim when no local SAS had been derived yet. They may now only
  corroborate the locally derived code.

- Anti-replay never ran. The sequence-number and AAD validators were defined on
  SecureKeyStorage instead of the connection manager, so every call site failed
  with a TypeError and the sliding replay window was dead code. Moved onto the
  manager, wired into the live chat path, and a missing or non-numeric sequence
  number now fails closed instead of sailing through the range checks.

- File transfers are gated on verification in both directions. Control frames are
  written straight to the data channel by the transfer system; sending was
  already gated, receiving now is too.

- Tighter CSP: connect-src and img-src no longer allow arbitrary https: hosts
  (nothing in the app talks to a third party), plus base-uri 'none'.

- The SAS is no longer written to logs, and is compared in constant time on every
  path. Fixed SecureMasterKeyManager.isUnlocked() testing a field renamed long
  ago, so it never actually gated anything.

- Fixed the header showing "Secure undefined%": getRealSecurityLevel() became
  reachable for the first time by the move above and returned only per-feature
  booleans, while the header renders `level` and `score` directly. It now runs
  the same verified scoring as every other consumer.

Adds regression tests for the verification gate, inbound frame authentication and
the security-level shape.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.5.0",
+  "version": "5.5.2",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {
@@ -11,7 +11,7 @@
     "dev": "npm run build && python -m http.server 8000",
     "watch": "npx tailwindcss -i src/styles/tw-input.css -o assets/tailwind.css --watch",
     "serve": "npx http-server -p 8000",
-    "test": "node tests/sas-verification.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs"
+    "test": "node tests/sas-verification.test.mjs && node tests/verification-gate.test.mjs && node tests/inbound-frame-authentication.test.mjs && node tests/security-level-shape.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs"
   },
   "keywords": [
     "p2p",