feat(ui): Chat tab — channel filters + display prefs + font slider

Phase L.0 (cont.) — fourth tab on the Settings shell. Mixes retail's
CharacterOptions2 chat-channel filter bits (Hear*Chat / TimeStamp /
FilterLanguage / AppearOffline) with a font-size slider that has no
retail bitfield equivalent.

ChatSettings record (9 fields):
 · 5 channel filters: HearGeneralChat, HearTradeChat, HearLFGChat,
   HearRoleplayChat, HearSocietyChat
 · 3 display flags: ShowTimestamps, FilterProfanity, AppearOffline
 · 1 visual: FontSize (10..20 pt)

Local-only this phase per the brainstorm — Hear*Chat flags affect
client-side display filtering only; the server still streams every
channel. Server-sync arrives later when the protocol round-trip is
in place.

SettingsStore grows LoadChat / SaveChat using the existing generic
SaveSection helper. All four non-keybind sections (display, audio,
gameplay, chat) now coexist non-destructively in settings.json.

SettingsVM grows the parallel chat state machine. HasUnsavedChanges,
Save, Cancel, ResetAllToDefaults all cover chat. Constructor signature
adds two more params; existing call sites updated.

SettingsPanel.RenderChatTab replaces the L.0-shell placeholder —
8 Checkbox calls grouped under "Channel filters" + "Display"
headers, plus a font-size SliderFloat. The "Coming soon" placeholder
test was retargeted from "Chat" to "Character" since Chat is no
longer a placeholder.

GameWindow wires SettingsStore.LoadChat / SaveChat + a TODO comment
for the future ChatPanel filter integration (read SettingsVM.ChatDraft
when filtering inbound chat lines).

13 new tests:
 · ChatSettings record (3) — defaults pinned, value equality, with-
   expressions
 · SettingsStore chat (3) — missing-file → defaults, round-trip, all
   four sections coexist
 · SettingsVM chat (5) — initial draft, SetChat marks dirty, Save
   invokes callback, Cancel reverts, ResetAllToDefaults covers
 · SettingsPanel chat tab (2) — checkboxes + slider render only when
   active

dotnet build green (0 warnings); dotnet test 1,289 / 1,289 green
(243 Core.Net + 373 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.Abstractions/Panels/Settings/SettingsPanel.cs

@@ -98,7 +98,7 @@ public sealed class SettingsPanel : IPanel
             }
             if (renderer.BeginTabItem("Chat"))
             {
-                RenderPlaceholder(renderer, "Chat");
+                RenderChatTab(renderer);
                 renderer.EndTabItem();
             }
             if (renderer.BeginTabItem("Character"))
@@ -356,6 +356,67 @@ public sealed class SettingsPanel : IPanel
             + "follow-up phase.");
     }
 
+    /// <summary>
+    /// Render the Chat tab — channel filters (Hear*Chat), display
+    /// preferences (timestamps / profanity filter / appear offline),
+    /// and a font-size slider. Channel filters affect client-side
+    /// display only this phase — the server still sends every line,
+    /// the client decides what to render.
+    /// </summary>
+    private void RenderChatTab(IPanelRenderer renderer)
+    {
+        var c = _vm.ChatDraft;
+
+        renderer.Text("Channel filters");
+        renderer.Separator();
+
+        bool general = c.HearGeneralChat;
+        if (renderer.Checkbox("General", ref general))
+            _vm.SetChat(c with { HearGeneralChat = general });
+
+        bool trade = c.HearTradeChat;
+        if (renderer.Checkbox("Trade", ref trade))
+            _vm.SetChat(c with { HearTradeChat = trade });
+
+        bool lfg = c.HearLFGChat;
+        if (renderer.Checkbox("LFG (looking for group)", ref lfg))
+            _vm.SetChat(c with { HearLFGChat = lfg });
+
+        bool rp = c.HearRoleplayChat;
+        if (renderer.Checkbox("Roleplay", ref rp))
+            _vm.SetChat(c with { HearRoleplayChat = rp });
+
+        bool society = c.HearSocietyChat;
+        if (renderer.Checkbox("Society (CD / EW / RB)", ref society))
+            _vm.SetChat(c with { HearSocietyChat = society });
+
+        renderer.Spacing();
+        renderer.Text("Display");
+        renderer.Separator();
+
+        bool timestamps = c.ShowTimestamps;
+        if (renderer.Checkbox("Show timestamps", ref timestamps))
+            _vm.SetChat(c with { ShowTimestamps = timestamps });
+
+        bool profanity = c.FilterProfanity;
+        if (renderer.Checkbox("Filter profanity", ref profanity))
+            _vm.SetChat(c with { FilterProfanity = profanity });
+
+        bool offline = c.AppearOffline;
+        if (renderer.Checkbox("Appear offline (hide from /who)", ref offline))
+            _vm.SetChat(c with { AppearOffline = offline });
+
+        float fontSize = c.FontSize;
+        if (renderer.SliderFloat("Font size (pt)", ref fontSize, 10f, 20f))
+            _vm.SetChat(c with { FontSize = fontSize });
+
+        renderer.Spacing();
+        renderer.TextWrapped(
+            "Channel filters hide messages from the chat window without "
+            + "changing your server-side subscriptions. Save persists; "
+            + "Cancel reverts.");
+    }
+
     private void RenderSection(IPanelRenderer renderer, string label, InputAction[] actions)
     {
         // Movement defaults open; other sections collapsed for first-run UX.