fix: 5 bugs critiques de l'éditeur (Phase 1 audit)

1. replaceAll (Find & Replace) — une seule transaction ProseMirror
   au lieu d'un forEach cassé. Tous les matchs sont maintenant remplacés.

2. Link Preview unwrap — deleteNode() au lieu de clearer les attrs
   qui laissaient un nœud fantôme invisible dans le document.

3. Conversion Markdown → richtext — breaks: true dans marked.parse()
   Les simple newlines sont maintenant convertis en <br>.
   + préserve les blocs custom (toggle, callout, math, columns,
   outline, link-preview) en commentaires HTML lors de l'export MD.

4. emitNoteChange exercices — shape corrigée (type:'created' attend
   un objet Note, pas noteId/notebookId séparés).

5. Raccourcis clavier sans conflit :
   Cmd+Shift+C → Cmd+Alt+C (callout, avant: copier)
   Cmd+Shift+O → Cmd+Alt+O (outline, avant: historique/signets)
   Cmd+Shift+L → Cmd+Alt+L (colonnes, avant: lock screen macOS)

memento-note/lib/editor/markdown-export.ts

@@ -121,7 +121,7 @@ interface BlockPlaceholder {
 function preprocessCustomNodes(html: string): { html: string; placeholders: BlockPlaceholder[] } {
   const placeholders: BlockPlaceholder[] = []
 
-  // liveBlock: <div data-live-block="true" sourceNoteId="..." blockId="..."></div>
+  // liveBlock
   let result = html.replace(
     /<div([^>]*?data-live-block[^>]*?)>\s*<\/div>/gi,
     (_match, attrs) => {
@@ -133,7 +133,7 @@ function preprocessCustomNodes(html: string): { html: string; placeholders: Bloc
     }
   )
 
-  // structuredViewBlock: <div data-structured-view-block="true" ...></div>
+  // structuredViewBlock
   result = result.replace(
     /<div([^>]*?data-structured-view-block[^>]*?)>\s*<\/div>/gi,
     (_match, attrs) => {
@@ -149,6 +149,71 @@ function preprocessCustomNodes(html: string): { html: string; placeholders: Bloc
     }
   )
 
+  // toggleBlock — preserve as HTML comment
+  result = result.replace(
+    /<div([^>]*?data-type="toggle-block"[^>]*?)>([\s\S]*?)<\/div>/gi,
+    (_match, attrs, content) => {
+      const opened = attrs.match(/data-opened="([^"]*)"/i)?.[1] || 'true'
+      const key = `${SENTINEL_PREFIX}TOGGLE${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- toggle: opened=${opened} -->${content}\n<!-- /toggle -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // calloutBlock
+  result = result.replace(
+    /<div([^>]*?data-type="callout-block"[^>]*?)>([\s\S]*?)<\/div>/gi,
+    (_match, attrs, content) => {
+      const type = attrs.match(/data-callout-type="([^"]*)"/i)?.[1] || 'info'
+      const key = `${SENTINEL_PREFIX}CALLOUT${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- callout: type=${type} -->${content}\n<!-- /callout -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // mathEquationBlock
+  result = result.replace(
+    /<div([^>]*?data-type="math-equation"[^>]*?)>([\s\S]*?)<\/div>/gi,
+    (_match, attrs, content) => {
+      const latex = attrs.match(/data-latex="([^"]*)"/i)?.[1] || content.trim()
+      const key = `${SENTINEL_PREFIX}MATH${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- math: ${latex} -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // outlineBlock
+  result = result.replace(
+    /<div[^>]*data-type="outline-block"[^>]*><\/div>/gi,
+    () => {
+      const key = `${SENTINEL_PREFIX}OUTLINE${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- outline -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // columns
+  result = result.replace(
+    /<div([^>]*?data-type="columns"[^>]*?)>([\s\S]*?)<\/div>/gi,
+    (_match, attrs, content) => {
+      const cols = attrs.match(/cols="([^"]*)"/i)?.[1] || '2'
+      const key = `${SENTINEL_PREFIX}COLUMNS${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- columns: cols=${cols} -->${content}\n<!-- /columns -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // linkPreviewBlock
+  result = result.replace(
+    /<div([^>]*?data-type="link-preview-block"[^>]*?)>[\s\S]*?<\/div>/gi,
+    (_match, attrs) => {
+      const url = attrs.match(/data-url="([^"]*)"/i)?.[1] || ''
+      const key = `${SENTINEL_PREFIX}LINKPREVIEW${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- link-preview: ${url} -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
   return { html: result, placeholders }
 }
 
@@ -188,10 +253,10 @@ export function tiptapHTMLToMarkdown(html: string): string {
 export function markdownToHTML(markdown: string): string {
   if (!markdown || markdown.trim() === '') return ''
 
-  // marked v18+ uses synchronous parse by default when no async tokens
+  // breaks: true — single \n becomes <br>, matching WYSIWYG expectations
   const html = marked.parse(markdown, {
     gfm: true,
-    breaks: false,
+    breaks: true,
   }) as string
 
   return html