Gamify: E-P5 'try this next' discovery nudges

[fernandotonon]

Part of #796 — Gamify: editor half. Depends on E-P1/E-P4.

Goal

Actively help users discover the editor's depth: surface unused feature clusters as gentle "try this next" suggestions — the core of the "discover more features" goal.

Behavior

• From GET /v1/me/stats (featureUsage), compute the set of clusters the user hasn't tried.
• On the welcome screen (WelcomeScreenController / WelcomeDialog): show one rotating, dismissible suggestion card — "New to you: Auto-Rig — turn a static mesh into a rigged character" with a button that opens the relevant tool/doc.
• Rotate suggestions; respect "don't show again"; never modal, never blocking. Cap at one card.
• Bias suggestions toward features adjacent to what the user already does (e.g. if they rig, suggest skin-weight painting) when that signal is cheap; otherwise round-robin.

Acceptance criteria

• Welcome screen shows at most one dismissible "try this" card for an unused cluster.
• Suggestions never repeat a feature already in featureUsage.
• "Don't show again" and per-card dismiss are honored and persisted.
• Logged-out → generic feature highlights (no personalization) or hidden, per setting.
• No modal/blocking behavior; reduced-motion-safe.