test(ui): add unit test coverage for Tabs and Avatar/AvatarStack components (#277, #278)

[ghzhost]

What this does

Resolves #277 and #278 by introducing dedicated unit test coverage for Tabs and Avatar/AvatarStack UI components:

1. Tabs component (src/components/ui/Tabs.test.tsx) (Tabs has no test file — its click-handling and active-tab-switching logic (independent of its known ARIA gaps) is entirely unverified #277):

   • Asserts that all tab labels render correctly.
   • Asserts clicking a tab triggers onChange with the clicked tab's key.
   • Asserts active tab receives active styling (bg-white text-slate-900) while inactive tabs receive text-slate-500.
   • Asserts count badges render if and only if typeof tab.count === "number" (including 0).

2. Avatar / AvatarStack component (src/components/ui/Avatar.test.tsx) (Avatar and AvatarStack have no test file — AvatarStack's overflow-count ("+N") calculation is entirely unverified #278):

   • Asserts dicebear URL construction with seed encoding and data-unoptimized flag.
   • Asserts explicit src override handling.
   • Asserts AvatarStack overflow calculations for cases:
     • Under max seeds (all rendered, no overflow badge).
     • Exactly at max seeds (rest === 0, no overflow badge).
     • Over max seeds (renders max avatars + exact +N badge).

How I verified it

• Executed full project build (npm run build).
• TypeScript type checking and Next.js compiler verification passed cleanly.

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