Files tab: .md → note editor via CheckFileAction + frontend

Backend:
- FindByFileID: notes+files JOIN query
- LinkFile: INSERT OR IGNORE notes record
- CheckFileAction binding: note/preview/external/auto-link

Frontend (App.svelte):
- Import isMarkdownFile from fileUtils
- openPreview now calls CheckFileAction for .md files
- .md+note → switch to Notes tab + note editor
- .md outside Notes/ → inline preview
- non-.md → unchanged

Tests: 7 new (FindByFileID×3, CheckFileAction×4), all PASS

frontend/src/App.svelte

@@ -14,7 +14,7 @@
   import AppHeader from './lib/AppHeader.svelte'
   import { onMount, onDestroy } from 'svelte'
   import { actionIcon } from './lib/actionIcons.js'
-  import { canPreviewFile, needsBase64Preview, needsTextPreview } from './lib/fileUtils.js'
+  import { canPreviewFile, needsBase64Preview, needsTextPreview, isMarkdownFile } from './lib/fileUtils.js'
   import { t } from './lib/i18n'
   import NoteEditorPanel from './lib/components/notes/NoteEditorPanel.svelte'
   import InternalLinkPicker from './lib/components/notes/InternalLinkPicker.svelte'
@@ -622,6 +622,24 @@
   // ===== File preview =====
 
   async function openPreview(item) {
+    // For .md files: check if linked to a note, open note editor instead of preview modal
+    if (item && item.fileId && isMarkdownFile(item)) {
+      try {
+        const action = await wailsCall('CheckFileAction', item.fileId)
+        if (action.action === 'note') {
+          await openNote({ id: action.noteId, title: action.noteTitle })
+          return
+        }
+        if (action.action === 'external') {
+          await wailsCall('OpenFile', item.fileId)
+          return
+        }
+        // 'preview' → fall through to normal preview
+      } catch (e) {
+        console.warn('CheckFileAction failed, falling back to preview:', e)
+      }
+    }
+
     previewItem = item
     previewContent = ''
     previewError = ''