[cuegui] Fix layer tags validation to allow dashes and underscores #2120
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Summarize your change.
The TagsWidget.get_tags() method used isalnum() to filter tags, which rejected any tag containing dashes (e.g., "test-layer-tag"). This caused the layer properties dialog to fail with "no tag selected" error.
Replace isalnum() with a new _is_valid_tag() method that uses regex to allow alphanumeric characters, dashes, and underscores in tag names.