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/ChatSettings.cs

@@ -0,0 +1,44 @@
+namespace AcDream.UI.Abstractions.Panels.Settings;
+
+/// <summary>
+/// Chat-related preferences persisted to <c>settings.json</c>. Mixes
+/// retail's <c>CharacterOptions2</c> chat-channel filter bits (Hear*Chat
+/// + TimeStamp + FilterLanguage + AppearOffline) with a few visual
+/// preferences (font size) that don't have a retail bitfield.
+/// See <c>docs/research/named-retail/acclient.h:3451+</c> for the
+/// retail bit values.
+///
+/// <para>
+/// L.0 scope: <b>local-only</b> like the rest of L.0. The Hear*Chat
+/// flags affect client-side <i>display</i> filtering of the existing
+/// channels — the server still streams every line; the client decides
+/// what to render. Server-sync arrives in a later phase that flips the
+/// retail-faithful "tell server which channels I'm subscribed to"
+/// switch.
+/// </para>
+/// </summary>
+public sealed record ChatSettings(
+    // CharacterOptions2 (32-bit) channel filters.
+    bool HearGeneralChat,                // 0x100   — General channel
+    bool HearTradeChat,                  // 0x200   — Trade channel
+    bool HearLFGChat,                    // 0x400   — LFG channel
+    bool HearRoleplayChat,               // 0x800   — RP channel
+    bool HearSocietyChat,                // 0x80000 — Society chat (CD/EW/RB)
+    bool AppearOffline,                  // 0x1000  — hide /who status
+    bool ShowTimestamps,                 // 0x40    — TimeStamp prefix on chat lines
+    bool FilterProfanity,                // 0x20000 — FilterLanguage (Turbine's profanity filter)
+    // Visual / UX (no retail bitfield).
+    float FontSize)                      // chat panel font, 10..20 pt
+{
+    /// <summary>Sensible starting values matching the retail "all on" stance.</summary>
+    public static ChatSettings Default { get; } = new(
+        HearGeneralChat:    true,
+        HearTradeChat:      true,
+        HearLFGChat:        true,
+        HearRoleplayChat:   true,
+        HearSocietyChat:    true,
+        AppearOffline:      false,
+        ShowTimestamps:     true,
+        FilterProfanity:    true,
+        FontSize:           12f);
+}