fix undecryptable self-copies by preserving the self-session

- Purge the poisoned self-session only once per account instead of on
  every connect, and rebuild it from the own published bundle when
  missing: deleting it repeatedly broke encrypt-to-self, so archived
  copies of own messages (most visibly in the self-chat) failed with
  "could not decrypt".
- Detect OMEMO 2 (urn:xmpp:omemo:2) payloads, which the pinned
  MartinOMEMO does not implement, and show them as undecryptable
  instead of empty plaintext bubbles.
- Guard both behaviours in verify.sh and document the namespace
  limitation in SECURITY.md.

UITests/TimelineUITests.swift

@@ -46,11 +46,14 @@ final class TimelineUITests: XCTestCase {
         let app = launchApp()
         openChat(app)
 
-        // Message 60 can sit partially behind the composer, so drag on a
-        // bubble that is reliably visible and hittable in the bottom area.
-        let target = bubble("uitest-msg-55", in: app)
-        XCTAssertTrue(target.waitForExistence(timeout: 10))
-        XCTAssertTrue(target.isHittable, "The swipe target must be hittable")
+        // The initial scroll position varies slightly between runs, so pick
+        // the first hittable bubble from the bottom area of the timeline.
+        let candidates = [60, 59, 58, 55, 50, 45].map {
+            bubble("uitest-msg-\($0)", in: app)
+        }
+        let target = candidates.first { $0.exists && $0.isHittable }
+        XCTAssertNotNil(target, "At least one swipe target must be hittable")
+        guard let target else { return }
         // A controlled right-to-left drag: the stock swipeLeft() flicks too
         // fast for the gesture's horizontal lock to engage reliably.
         let start = target.coordinate(withNormalizedOffset: CGVector(dx: 0.9, dy: 0.5))