Add placeholder text to the New issue description field

Problem

The Description box on the New issue page is blank. Nothing hints at what to write or that Markdown is supported.

Expected behavior

Show placeholder text in the empty description field, for example: "Describe the issue: what happened, what you expected, steps to reproduce… Markdown is supported."

Acceptance criteria

• The placeholder appears while the field is empty and disappears as soon as the user types.
• The field keeps its visible label. The placeholder is a hint, not the label.
• Placeholder text meets contrast guidelines in light and dark modes.
• Other description editors that start empty (e.g. sub-issue creation) get equivalent placeholder text for consistency.