Add difficulty modes, interference variants, dynamic bins, and improved purity checks/UI

[ancient1110]

Motivation

• Introduce adjustable difficulty and richer visual interference to make the sample-sorting game more varied and challenging.
• Improve real-time purity feedback, remove deprecated hint/round UI, and present a clearer success banner when a level is perfectly solved.

Description

• Add difficulty configuration and UI: a difficultyBar with buttons and functions setDifficulty / refreshSamples to drive difficultyConfig (easy/normal/hard/expert) and update currentLevel display.
• Extend sample model with multiple interference options and generator applyInterference, plus interferenceOptions and pickSome helpers to apply random tintShift, morph, and size classes to visuals.
• Make bins dynamic via renderBins(binCount), derive binCount from difficulty, and change sample generation to use refreshSamples that populates state.samples and starts animation.
• Refine purity and scoring logic in checkAnswer to only mark actually mixed bins as warnings, compute correct rate per-card, show a perfectBanner overlay on full success, and award fixed score on perfect grouping; remove previous giveHint, round/streak/log UI elements.
• Update visuals and CSS to support new interference classes, active difficulty button styles, perfectOverlay, and adjusted sample/bin animations and styles.

Testing

• No automated tests were run for this change.

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Codex Task