Add success feedback for user actions

Background

Split from #71 (now closed as outdated).

Problem

When users complete actions we want them to take (submitting evaluations, saving preferences, etc.), there's no satisfying feedback. The UI silently advances or updates, which misses an opportunity to reinforce positive behavior and make the experience feel rewarding.

Proposed Solution

Add subtle, delightful feedback for successful user actions:

• Visual confirmation when actions complete (animations, button states, micro-interactions)
• Reinforces the gamified session progress
• Makes the evaluation loop feel rewarding

This is about motivation and UX delight - helping users feel good about contributing.

Scope

Any action we want users to take and feel good about, including but not limited to:

• Submitting evaluations
• Saving preferences
• Completing a session goal

No files, tests, or entry points are identified. Start by locating the evaluation submission, preference-saving, and session-goal completion flows; define the feedback states and confirm each listed action visibly acknowledges success without disrupting progress.