feat(editor): inline spoiler mark (blur + click-reveal, lossless Markdown) (#246)

Add an inline spoiler (Telegram/Discord-style hidden text): a TipTap mark
`spoiler` rendered as <span data-spoiler="true" class="spoiler">, blurred via
CSS and revealed on click (UI-only is-revealed class, never persisted).

- packages/editor-ext: the Spoiler mark (inclusive:false, set/toggle/unset
  commands, ||text|| input rule), exported; a lossless turndown rule emitting
  raw inline HTML; round-trip test.
- apps/client: SpoilerView mark-view (ReactMarkViewRenderer, Link pattern),
  registration in extensions, bubble-menu toggle button (editable only), CSS
  (blur + @media print reveal), en/ru i18n.
- apps/server: register Spoiler in collaboration.util tiptapExtensions so the
  mark survives HTML<->JSON export/index/import/Yjs; a test proving the public
  share keeps the spoiler (it isn't stripped with comments).

No keyboard shortcut: the proposed Mod-Shift-s collides with Strike (and
Mod-Shift-h with Highlight); the ||text|| input rule + the bubble-menu button
cover ergonomics.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/extensions/extensions.ts

@@ -53,6 +53,7 @@ import {
   Subpages,
   Heading,
   Highlight,
+  Spoiler,
   Indent,
   UniqueID,
   SharedStorage,
@@ -116,6 +117,7 @@ import mentionRenderItems from "@/features/editor/components/mention/mention-sug
 import { ReactNodeViewRenderer, ReactMarkViewRenderer } from "@tiptap/react";
 import MentionView from "@/features/editor/components/mention/mention-view.tsx";
 import LinkView from "@/features/editor/components/link/link-view.tsx";
+import SpoilerView from "@/features/editor/components/spoiler/spoiler-view.tsx";
 import i18n from "@/i18n.ts";
 import { MarkdownClipboard } from "@/features/editor/extensions/markdown-clipboard.ts";
 import EmojiCommand from "./emoji-command";
@@ -237,6 +239,11 @@ export const mainExtensions = [
   Highlight.configure({
     multicolor: true,
   }),
+  Spoiler.configure({}).extend({
+    addMarkView() {
+      return ReactMarkViewRenderer(SpoilerView);
+    },
+  }),
   Typography,
   TrailingNode,
   GlobalDragHandle.configure({