fix: restore textarea resize handle visibility

[EurFelux]

What this PR does

Before this PR:
All <textarea> elements had their resize handle (the drag indicator in the bottom-right corner) hidden by a global CSS rule textarea::-webkit-resizer { display: none; }. The resize functionality itself still worked, but without the visual handle, users had no way to discover that textareas were resizable — a UX discoverability issue.

After this PR:
The global WebKit resizer hiding rule is removed, restoring the native drag-to-resize handle on textareas (e.g., MCP Server Settings - Parameters/Environment Variables). Users can now clearly see the resize indicator and know they can drag to expand text boxes.

Fixes #11698

Why we need it and why it was done in this way

The global textarea::-webkit-resizer { display: none; } rule in index.css was blanket-hiding all textarea resize handles across the app. Removing this single rule restores the native visual indicator for all textareas that have resize enabled.

Components that intentionally disable resize (e.g., the main chat input in InputbarCore.tsx and MessageEditor.tsx) already have their own resize: none !important styles, so they are unaffected by this change.

The following tradeoffs were made:

• Removing a global rule means all textareas without explicit resize: none will now show the handle. This is the intended behavior per the issue request.

The following alternatives were considered:

• Scoping the fix to only MCP settings textareas, but the global hide had no clear justification and removing it is cleaner.

Breaking changes

None. Components that explicitly set resize: none are unaffected.

Special notes for your reviewer

This is a CSS-only change — one rule removed from src/renderer/src/assets/styles/index.css.

Checklist

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required.

Release note

Restored the native resize handle on textarea elements, allowing users to visually discover and use the drag-to-resize feature on text boxes (e.g., MCP Server settings Parameters/Environment Variables).