Add Avatar test coverage, runtime status validation, and a11y fixes

[brightpixel-dev]

Closes #278
Closes #279
Closes #280
Closes #281

What changed

• Added Avatar.test.tsx covering Avatar render (src fallback, dicebear URL, size, className) and AvatarStack overflow counting (under/at/over max)
• Added coerceStatus() to validate raw bounty status strings against known BountyStatus values, falling back to "open" for unrecognized values
• adaptBounty now uses coerceStatus instead of passing raw.status directly
• MilestoneFundButton accepts optional milestoneName prop, sets aria-label "Fund milestone: {name}"
• PoolDepositButton accepts optional poolRepo prop, sets aria-label "Deposit to pool: {repo}"
• Fixed heading hierarchy on /issues page: inserted sr-only h2 "Available bounties" between h1 and BountyCard h3's

Why

• Avatar and AvatarStack have no test file — AvatarStack's overflow-count ("+N") calculation is entirely unverified #278: Avatar/AvatarStack had zero test coverage
• adaptBounty passes an unrecognized backend status straight through with no validation, silently breaking both the status badge and every action button #279: adaptBounty passed raw.status through without validation — a malformed backend response could produce an invalid BountyStatus at runtime
• MilestoneActions' Fund/Deposit buttons have no distinguishing accessible name across multiple milestones/pools, so screen-reader users can't tell which button is which out of context #280: Action buttons had no accessible names for screen readers
• BountyCard's h3 title skips directly from the page's h1 with no intervening h2, breaking heading-level navigation on /issues #281: /issues page skipped from h1 directly to h3, violating WCAG heading hierarchy

How to test

• npx jest src/components/ui/Avatar.test.tsx — all 13 tests pass
• npx jest src/components/ui/Badge.test.tsx — all 27 tests pass
• Verify MilestoneFundButton renders with aria-label when milestoneName is provided
• Verify heading order on /issues: h1 > h2 (sr-only) > h3 (BountyCard)

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