aUiux/review moderation flag modal

[code-0-stella]

Moderation modal

The new accessible review moderation flag modal is now in place with:

• a dialog pattern using focus trapping and Escape handling
• reason selection with an optional context textarea
• inline validation feedback when no reason is chosen
• confirmation feedback for successful submission and already-flagged reviews
• test coverage for rendering, submission, close behavior, and validation

Closes: #263

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