fix(elements): clamp context usage percentage to valid range

[ephraimduncan]

Problem

Context computes usedTokens / maxTokens unguarded in three places (ContextIcon, ContextTrigger, ContextContentHeader):

• maxTokens={0} with usedTokens={0} renders NaN% and passes NaN as the ring's strokeDashoffset
• maxTokens={0} with positive usage renders ∞%
• Over-budget usage (e.g. 150/100) renders 150%, overdraws the ring, and the Progress indicator's translateX(-${100 - value}%) becomes translateX(50%) — the bar looks empty exactly when the context is most full
• Negative token values render outside the meaningful range

Fix

One shared clampUsedPercent helper applied to all three readouts:

• non-finite input or maxTokens <= 0 → stable 0% empty state
• otherwise the ratio clamped to [0, 1], so over-budget usage shows a full ring/bar instead of visually resetting

No transitions added — high-frequency streaming updates stay immediate.

Tests

Extended __tests__/context.test.tsx with the degenerate cases (0,0), (50,0), (-1,100) (bounded 0% text, empty ring) and the over-budget case (150,100) (100% text, zero dash offset, full progress bar). All 31 tests pass.

[vercel]

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