feat: auto-expanding chat text input

[comppaz]

Component

Multiple / Not sure

Problem / Motivation

The chat input field remains as a single line, making it difficult to review longer messages before sending. Users have to scroll horizontally or can't see their full message.

Reference: #48

Proposed Solution

Convert the chat input from a fixed single-line <input> to an auto-expanding <textarea> that:

• Starts as a single line (44px min height)
• Expands dynamically as users type
• Caps at ~8 lines (200px max height) with scroll
• Enter submits, Shift+Enter adds new line

Alternatives Considered

Could use a fixed multi-line textarea, but dynamic expansion provides better UX by not taking up space when not needed.

Additional Context

Affects both the desktop app (ChatInput.tsx) and browser extension (ChatSidebar.tsx).