Replace the comma-separated text input for tags in the metadata editor
with a badge-based tag picker:
- Current tags shown as removable pill badges (✕ button per tag)
- Autocomplete input queries existing tags via shared tag-dropdown module
- Results shown as inline dropdown (not absolute, avoids overflow clipping
from the modal's overflow-y-auto content area)
- Keyboard navigation: ArrowUp/Down, Enter to select, comma to add new,
Escape to close
- Supports adding new tags not in the database (type + Enter/comma)
- Hidden data-editor-tag spans seed initial tags from server-side render
- collectFormData() now accepts editorTags array, skips the removed
tags text input