feat(web): improve list auto-completion and tag insertion UX

Two user experience improvements based on feedback:

1. **GitHub-style empty list item handling:**
   - When pressing Enter on an empty list item (e.g., just "- "), remove
     the list marker instead of continuing the list
   - Matches GitHub's behavior for exiting list mode
   - Works for all list types: unordered (-, *, +), ordered (1., 2.),
     and task lists (- [ ])
   - Makes it easier to exit list mode without manual deletion

2. **Auto-space after tag selection:**
   - When selecting a tag from suggestions, automatically add a trailing
     space after the tag
   - Improves typing flow - users can immediately continue typing
   - Matches common expectation for autocomplete behavior

These changes make the editor feel more intuitive and reduce friction
during content creation.

web/src/components/MemoEditor/Editor/TagSuggestions.tsx

@@ -43,9 +43,9 @@ const TagSuggestions = observer(({ editorRef, editorActions }: TagSuggestionsPro
       return items.filter((tag) => tag.toLowerCase().includes(searchQuery));
     },
     onAutocomplete: (tag, word, index, actions) => {
-      // Replace the trigger word with the complete tag
+      // Replace the trigger word with the complete tag and add a trailing space
       actions.removeText(index, word.length);
-      actions.insertText(`#${tag}`);
+      actions.insertText(`#${tag} `);
     },
   });
 

web/src/components/MemoEditor/Editor/useListAutoCompletion.ts

@@ -52,8 +52,28 @@ export function useListAutoCompletion({ editorRef, editorActions, isInIME }: Use
 
       if (listInfo.type) {
         event.preventDefault();
-        const continuation = generateListContinuation(listInfo);
-        actions.insertText("\n" + continuation);
+
+        // Check if current list item is empty (GitHub-style behavior)
+        // Extract the current line
+        const lines = contentBeforeCursor.split("\n");
+        const currentLine = lines[lines.length - 1];
+
+        // Check if line only contains list marker (no content after it)
+        const isEmptyListItem =
+          /^(\s*)([-*+])\s*$/.test(currentLine) || // Empty unordered list
+          /^(\s*)([-*+])\s+\[([ xX])\]\s*$/.test(currentLine) || // Empty task list
+          /^(\s*)(\d+)[.)]\s*$/.test(currentLine); // Empty ordered list
+
+        if (isEmptyListItem) {
+          // Remove the empty list marker and exit list mode
+          const lineStartPos = cursorPosition - currentLine.length;
+          actions.removeText(lineStartPos, currentLine.length);
+          actions.insertText("\n");
+        } else {
+          // Continue the list with the next item
+          const continuation = generateListContinuation(listInfo);
+          actions.insertText("\n" + continuation);
+        }
       }
     };