Add GnTagInput component for multi-value string input ...

Features:
- New GnTagInput Vue component with inline chip display
- Enter / paste (with separator split) to add tags
- Backspace in empty field removes last tag
- Unique check and maxItems limit
- Removable chips with x button

Style:
- Add _tag-input.scss with .tag-input, .tag-input-wrap, .tag-input-field
- Chips reuse existing .chip-secondary styling

Demo:
- Add interactive tag input example to Forms section (vanilla + Vue)
- Add code example blocks for HTML and Vue usage

Docs:
- Add GnTagInput to CLAUDE.md component catalog

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Replace code-example-copy with standard btn-icon ...

All copy buttons inside `.code-example` blocks now use `.btn-icon.btn-icon-sm`
with a Phosphor `ph-copy` icon instead of the old text-based `.code-example-copy`.

Updated `code-examples.js` to swap the icon to `ph-check` for 1.2 s after copy,
matching the behaviour of `GnCopyButton`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add date time form controls

Add file upload previews

Expand typography and forms

Generalize UI kit branding

Scaffold GNexus UI kit