Add friends, markers, HUD overlays, tab complete

- FriendsManager persists a per-user friend list at config/clientirc-friends.json.
- MarkerManager holds one ephemeral marker per owner (5 min expiry).
- IRC protocol extended with marker_set / marker_remove; server relays them.
- IrcService DisplayLine replaced with sealed IrcEvent (Chat / System / MarkerSet / MarkerRemove); chat formatting moved into the client.
- ClientIrcMod handles .friends add/remove/list, .marker remove, .help; chat lines from friends are prefixed with a green dot; marker events from non-friends are ignored.
- OverlayHud draws a top-center compass strip and a top-left watermark (nick / FPS / ping) via HudLayerRegistrationCallback.
- WorldOverlayRenderer draws green crosses above friends and magenta beams + base cubes at active markers via WorldRenderEvents.AFTER_ENTITIES.
- MiddleClickHandler binds MMB via KeyMapping, raycasts a block or 20-block look ray, stores locally and sends marker_set on a daemon executor.
- ChatScreenMixin completes . prefix commands on Tab; first mixin in the project, plumbed through clientirc.client.mixins.json and fabric.mod.json.

src/main/resources/fabric.mod.json

@@ -15,6 +15,9 @@
       "ru.nevetime.clientirc.ClientIrcMod"
     ]
   },
+  "mixins": [
+    { "config": "clientirc.client.mixins.json", "environment": "client" }
+  ],
   "depends": {
     "fabricloader": ">=0.19.3",
     "minecraft": "26.2",