feat: add MarkdownToolbar with format buttons, character counter, and image paste support (#399)

[waterWang]

Summary

Enhances the Markdown editor on the bounty creation form with a formatting toolbar, live character counter, and image paste support.

Changes

1. New MarkdownToolbar component (components/MarkdownToolbar.tsx):

   • Format buttons: Bold, Italic, Link, Code, Heading, Bullet List, Numbered List
   • Insert Image button with file picker (accepts PNG, JPG, GIF, WebP)
   • Live character counter with color-coded warning near limit (10,000 char cap)
   • Keyboard shortcut hint (Ctrl+P to toggle preview)
   • Image paste from clipboard (paste -> upload to /api/v1/upload -> fallback to base64 data URL)
   • Drag and drop image support
   • Dark mode compatible

2. Updated CreateBountyPage (app/bounties/new/page.tsx):

   • Integrated MarkdownToolbar into the Write tab
   • Added Ctrl+P keyboard shortcut to toggle Write/Preview tabs
   • Refactored textarea to use controlled component with setValue for toolbar integration
   • Improved editor border styling (rounded border wraps the entire editor group)

3. New tests (components/MarkdownToolbar.spec.tsx):

   • 9 unit tests covering button rendering, character count, formatting actions (bold, italic, link, code), and placeholder insertion

Acceptance Criteria

• Counter reflects text length as user types
• Toolbar: Bold, Italic, Link, Code buttons available
• Pasting a PNG uploads and inserts image markdown line (with API fallback to data URL)
• All existing tests pass (9/9)

Closes #399