Enhance changelog input to textarea with character counter

[shresthkapoor7]

This PR addresses #5434 by improving the UX of the changelog input field during page edits.

Changes:

• Replaced single-line <input> with compact <textarea> (rows="2", maxlength="250")
• Added live character counter (0 / 250) below the field
• Preserved preview logic using existing debounced JS handler
• UI remains visually compact as requested

Tested:

• Page creation and editing flow
• Character counter updates live
• Summary preview still truncates after 16 chars
• Long changelogs saved correctly and appear in revision history

Notes:

• Revision list may overflow with long changelogs - possibly a separate CSS issue

[ssddanbrown]

Thanks @shresthkapoor7! Now merged to be part of the next feature release.