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/components/bubble-menu/bubble-menu.tsx

@@ -9,6 +9,7 @@ import {
   IconStrikethrough,
   IconUnderline,
   IconMessage,
+  IconEyeOff,
 } from "@tabler/icons-react";
 import clsx from "clsx";
 import classes from "./bubble-menu.module.css";
@@ -74,6 +75,7 @@ export const EditorBubbleMenu: FC<EditorBubbleMenuProps> = (props) => {
         isStrike: ctx.editor.isActive("strike"),
         isCode: ctx.editor.isActive("code"),
         isComment: ctx.editor.isActive("comment"),
+        isSpoiler: ctx.editor.isActive("spoiler"),
       };
     },
   });
@@ -109,6 +111,12 @@ export const EditorBubbleMenu: FC<EditorBubbleMenuProps> = (props) => {
       command: () => props.editor.chain().focus().toggleCode().run(),
       icon: IconCode,
     },
+    {
+      name: "Spoiler",
+      isActive: () => editorState?.isSpoiler,
+      command: () => props.editor.chain().focus().toggleSpoiler().run(),
+      icon: IconEyeOff,
+    },
   ];
 
   const commentItem: BubbleMenuItem = {