fix(ui): add missing ui.tool.skill i18n key; guard TextShimmer against undefined text

[Copilot]

The "STUPID SEXY TIMELINE" refactor (bbd0f3a25, Mar 7) changed the skill tool renderer from a hardcoded input.name || "skill" fallback to i18n.t("ui.tool.skill"), but never added the key to any locale dictionary. @solid-primitives/i18n returns undefined for missing keys; that undefined reached TextShimmer, which calls props.text.length in a reactive style computation — instant crash for every session that ran a skill tool call.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• packages/ui/src/i18n/en.ts + all 16 locale files — adds "ui.tool.skill" with appropriate translations (e.g. "Skill", "技能", "Compétence", etc.)
• packages/ui/src/components/text-shimmer.tsx — guards props.text with ?? "" in both shimmerSize and shimmerDuration memos so any future missing i18n key degrades gracefully instead of crashing

-  const len = Math.max(props.text.length, 1)
+  const len = Math.max((props.text ?? "").length, 1)

How did you verify your code works?

Traced the call path: bbd0f3a25 introduced i18n.t("ui.tool.skill") in the skill ToolRegistry renderer without defining the key → translator() returns undefined → passed as text to TextShimmer → props.text.length throws. Confirmed the key was absent from en.ts (and all locales) prior to this fix.

Screenshots / recordings

UI-only crash fix; no visual change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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