Console warning: Duplicate extension names found ['link'] in Tiptap editor

## Description

The browser console shows a warning on every editor mount:

```
[tiptap warn]: Duplicate extension names found: ['link']. This can lead to issues.
```

## Steps to Reproduce

1. Open the app
2. Open any markdown file
3. Check browser DevTools console
4. Warning appears every time the editor mounts

## Expected Behavior

No duplicate extension warnings in the console.

## Actual Behavior

Warning spam on every editor mount, and potential for unpredictable behavior when two extensions share the same name.

## Root Cause (Investigation)

StarterKit v3.x now includes the Link extension by default. The codebase also separately adds `Link.configure()` with custom settings (`openOnClick: false`, custom HTMLAttributes). This results in two Link extensions being registered.

## Impact

- Console warning spam
- Unpredictable behavior when two extensions share a name (Tiptap uses one and ignores the other non-deterministically)
- Configuration conflicts between StarterKit's Link defaults and the custom settings

## Environment

- Rowboat version: v0.1.60
- Tiptap version: 3.15.x
- OS: All platforms

Start by locating the editor extension setup that registers StarterKit and Link, then inspect how StarterKit v3 handles Link by default. Mount the editor and verify that the duplicate-extension warning no longer appears while the custom link behavior and HTML attributes remain effective.

Written by the indexing model from the issue text.