fix: fill the editor input area with the textarea - #713

Fixes #700.

With short content and a fixed editor height, the inline minHeight leaves the text layer shorter than the input pane. For example, height={408} gives a 379px input pane but only a 100px textarea. Clicking the blank lower area does not focus the editor, and the cursor changes to an arrow.

Make the input pane a column flex container and let the text layer grow without shrinking. This fills the editable area while preserving natural content height, scrolling, and the minimum height used by auto-height editors (#589).

The existing LESS variable declaration is also formatted to satisfy Prettier; its compiled CSS is unchanged.

Validation:

• CI=true npm test -- --runInBand - 56 tests passed.
• npm run build --workspace @uiw/react-md-editor - passed.
• npx --no-install prettier --check core/src/components/TextArea/index.less - passed.
• Playwright: 144 cases passed across Chromium, Firefox, and WebKit, comparing the same source with the original and patched stylesheet. Covers all three entrypoints, edit/live mode, auto height, long-content scrolling and typing, toolbar variants, fullscreen, custom textareas, and read-only/disabled inputs.

To reproduce, render <MDEditor height={408} preview="edit" value="Hello" />, then click near the bottom of the input pane and type. The patched version focuses the textarea and inserts the character; the original hits the surrounding div.