Group chat with E2E encryption for both web and CLI clients

Server:
- POST /v1/groups/create — create named group
- POST /v1/groups/:name/join — join group
- GET /v1/groups/:name — get group info + member list
- GET /v1/groups — list all groups
- POST /v1/groups/:name/send — fan-out encrypted messages to members
- Groups stored in sled, members tracked by fingerprint

Web client:
- /gcreate <name> — create group
- /gjoin <name> — join group
- /g <name> — switch to group chat mode
- /glist — list all groups
- /dm — switch back to DM mode
- Group messages encrypted per-member (ECDH + AES-GCM for each)
- Group tag shown on received messages: "sender [groupname]"

CLI TUI client:
- Same commands: /gcreate, /gjoin, /g, /glist, /dm
- Group messages encrypted per-member (X3DH + Double Ratchet for each)
- Automatic X3DH key exchange with new group members on first message
- Sessions established and persisted per-member

Architecture:
- Client-side fan-out encryption: message encrypted N times (once per member)
- Server stores one copy per recipient in their message queue
- Reuses existing 1:1 encryption — no new crypto primitives needed
- Works for groups ≤ 50 members (per DESIGN.md)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-server/src/db.rs

@@ -3,6 +3,7 @@ use anyhow::Result;
 pub struct Database {
     pub keys: sled::Tree,
     pub messages: sled::Tree,
+    pub groups: sled::Tree,
     _db: sled::Db,
 }
 
@@ -11,9 +12,11 @@ impl Database {
         let db = sled::open(data_dir)?;
         let keys = db.open_tree("keys")?;
         let messages = db.open_tree("messages")?;
+        let groups = db.open_tree("groups")?;
         Ok(Database {
             keys,
             messages,
+            groups,
             _db: db,
         })
     }