fix(a11y): fix heading hierarchy skips and add descriptive button aria-labels (#280, #281, #282)

[ghzhost]

Summary

Addresses accessibility and semantic heading hierarchy issues across the issues and milestones routes:

• Heading Hierarchy on /issues (BountyCard's h3 title skips directly from the page's h1 with no intervening h2, breaking heading-level navigation on /issues #281): Added an intermediate visually-hidden <h2> (Bounties list) before the bounty grid so screen readers navigating by heading level do not encounter a jump from <h1> directly to <h3>.
• Heading Hierarchy on /milestones (MilestonesPage's milestone card titles also skip from h1 directly to h3 with no h2 in between #282): Added an intermediate visually-hidden <h2> (Active milestones) before the milestones grid to maintain logical level order before reaching individual <h3> milestone titles.
• Accessible Names for Action Buttons (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): Passed milestone name and pool repo props to MilestoneFundButton and PoolDepositButton to provide explicit, context-rich aria-label attributes (aria-label={Fund ${milestoneName}} and aria-label={Deposit to ${poolRepo} pool}), allowing screen reader users navigating by interactive controls to distinguish buttons out of context.

Closes #280
Closes #281
Closes #282

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