i18n: extract hardcoded canvas-management strings to locale keys

[dalsoop]

Problem

The canvas-management UI added in this fork hardcodes Chinese strings instead of going through the existing i18n system, so non-Chinese users see Chinese text in an otherwise localized app:

• excalidraw-app/components/MyCreationsTab.tsx — empty-thumbnail placeholder 空空如也
• excalidraw-app/utils/time.ts — relative time (刚刚, {{n}} 秒前, 分钟前, 小时前, 天前)
• excalidraw-app/hooks/useCanvasManagement.ts — 4 login-required error messages (您需要登录才能…画布。)

Fix

Move these strings into the locale system via t():

• New keys under myCreations, timeAgo, canvasManagement.
• en.json gets English values (also the fallback / type source).
• zh-CN.json gets the original Chinese values, so behavior is unchanged for zh-CN users.
• All other locales now fall back to English instead of showing Chinese.

timeAgo.* use {{count}} interpolation, e.g. t("timeAgo.minutesAgo", { count }).

No functional change for zh-CN; this only stops leaking Chinese to non-Chinese locales.

[gemini-code-assist]

Code Review

This pull request externalizes hardcoded Chinese strings into translation keys across several files, including MyCreationsTab.tsx, useCanvasManagement.ts, and time.ts. It also adds the corresponding English and Chinese translation strings in the locale files. I have no additional feedback to provide as the changes are clean and correctly implemented.

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