feat: replace ASCII diagrams with mermaid in messaging deck

Signal Under the Hood: sequence diagram shows the message path and
what the server can/cannot see, clearer than ASCII art.

Matrix Federation: graph shows client→homeserver→federation topology,
replacing the ASCII box diagram.

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

docs/møder/2026-05-11_messaging-presentation.md

@@ -69,15 +69,18 @@ Non-profit Signal Foundation. The Signal Protocol powers WhatsApp, Google RCS, a
 
 ## Signal — Under the Hood
 
-```
-Alice's phone                 Signal Server              Bob's phone
-─────────────                 ─────────────              ──────────
-[message] ──encrypt(Bob)───▶ [stores ciphertext] ──────▶ decrypt ──▶ [message]
+```mermaid
+sequenceDiagram
+    participant A as Alice's phone
+    participant S as Signal Server
+    participant B as Bob's phone
+    A->>S: encrypted message
+    Note over S: sees: who, when, how often<br/>does NOT see: content
+    S->>B: encrypted message
+    Note over B: decrypts with private key
 ```
 
-- Server sees: _who_ talks to _whom_, _when_, _how often_
-- Server does **not** see: message content
-- Metadata still matters — [Signal subpoena responses](https://signal.org/bigbrother/)
+Metadata still matters — [Signal subpoena responses](https://signal.org/bigbrother/)
 
 ---
 
@@ -85,10 +88,11 @@ Alice's phone                 Signal Server              Bob's phone
 
 Matrix is a **protocol**, not an app — like email for real-time chat.
 
-```
-[your homeserver]  ←──federation──▶  [another homeserver]
-        ▲                                      ▲
-   Element client                        FluffyChat client
+```mermaid
+graph LR
+    EC[Element client] --> YH[your homeserver]
+    YH <-->|federation| OH[another homeserver]
+    FC[FluffyChat] --> OH
 ```
 
 - **Servers**: Synapse (Python), Conduit (Rust), Dendrite (Go)