i18n: replace hardcoded Chinese strings in multi-canvas dashboard with English

[dimafa]

What

Replaces the hardcoded Chinese strings in the multi-canvas dashboard with English, so non-Chinese self-hosters get a readable UI. There is currently no i18n or browser-locale path for these (they're baked into the compiled frontend).

Addresses BetterAndBetterII/excalidraw-full#20.

Changes

• excalidraw-app/utils/time.ts — relative timestamps: just now, N second(s)/minute(s)/hour(s)/day(s) ago (with simple pluralization).
• excalidraw-app/components/MyCreationsTab.tsx — empty thumbnail placeholder → No preview.
• excalidraw-app/hooks/useCanvasManagement.ts — the 4 AuthError toasts (delete / create / rename / save-as) → English. These now match the English non-auth fallback messages already present in the same catch blocks (e.g. "Could not delete the canvas.").

Notes

• Minimal, literal-only change (10 lines) — no behavior change, no new deps.
• Left the Chinese text-to-diagram system prompt in components/AI.tsx out of scope (it's an internal LLM prompt, not user-facing chrome) — happy to follow up if you'd like it Englished or i18n'd too.
• Ideally these would go through Excalidraw's t() i18n with locale files; this PR is the minimal first step toward an English-default. Glad to rework it into proper i18n keys if you prefer that approach.

[gemini-code-assist]

Code Review

This pull request localizes several user-facing strings from Chinese to English across the application, including placeholder text, error messages, and relative time-ago descriptions. In "time.ts", the pluralization check for seconds is redundant because the value is guaranteed to be at least 10 when reached; simplifying this to always use the plural form is recommended.

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[gemini-code-assist]

Since diffInSeconds is guaranteed to be at least 10 when this block is reached (due to the diffInSeconds < 10 check above), diffInSeconds can never be 1. The pluralization ternary diffInSeconds === 1 ? "" : "s" is redundant and will always evaluate to "s". We can simplify this string to always use the plural form.

Suggested change

	return `${diffInSeconds} second${diffInSeconds === 1 ? "" : "s"} ago`;
	return `${diffInSeconds} seconds ago`;

[dimafa]

Good catch — applied. The seconds branch can't reach a singular value (it's only hit at >= 10s), so I simplified it to a plain plural. Kept the ternary on the minutes/hours/days branches since those can legitimately be 1.