fix(miners): align issue search on mobile

[victortran0904]

Summary

• align the miner Issue Discovery search field with the table/card edges on xs viewports
• keep the existing tablet and desktop sizing unchanged

Addresses #867 on the current test branch. The original issue refers to the old Repositories tab/search label; that surface has since been refactored into the current Issue Discovery/Open Issues layout.

Verification

• npm run lint
• npm run build
• git diff --check
• visual check at 375px: /tmp/gittensor-867-after-375.png

Note: build passes with the existing Vite large chunk warning.

[Copilot]

Pull request overview

Updates the miner Issue Discovery “Open Issues” header search field styling so that, on extra-small (mobile) viewports, the search input aligns with the table/card edges while preserving existing sizing behavior on tablet/desktop.

Changes:

• Adjusted mobile (xs) horizontal spacing for the search TextField using negative margins.
• Expanded mobile (xs) width/maxWidth via calc() to visually align with the container edges without affecting sm+ sizing.

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