Add "My Issues only" filter to the Issues page

[cleanjunc]

Summary

The Issues page currently has no quick way to narrow results to issues opened by the signed-in user. The Pull Requests page already exposes a "My PRs only" toggle, so the same affordance should exist on /issues to keep both pages consistent.

Motivation

Finding personal issues across many tracked repos today requires typing a username into the author filter every visit. A dedicated toggle removes that friction and matches the workflow already established on the pulls page.

Proposed Solution

Add a "My Issues only" toggle next to the existing "Tracked only" button on /issues:

• Mirrors the attention-tone styling of "My PRs only" on /pulls
• Shows the count of issues opened by the signed-in user when greater than zero
• Syncs to the URL via ?mine=1 so the filter survives reloads and is shareable
• Mutually exclusive with the author dropdown (selecting an explicit author clears the toggle, and enabling the toggle overrides the dropdown)
• Highlights the author cell on rows opened by the signed-in user
• Falls back to an empty result for signed-out visitors via a sentinel author value, so no other account's data leaks through