[Omni] Fix popup Snap watermark response handling

[numbers-official]

Summary

Fixes popup-initiated Snap captures saving without a watermark. The popup runtime listener now ignores unrelated messages synchronously so the offscreen document can respond to ADD_WATERMARK while the popup remains open.

Changes

• Narrowed the popup chrome.runtime.onMessage handler to UPLOAD_PROGRESS only.
• Moved upload-progress async work into a fire-and-forget task so the listener does not return a Promise to the Chrome runtime.
• Kept selection overlay injection unchanged; no broad host permissions added.

Verification

• gh pr checks 84: ci pass
• npm run type-check: pass
• npm run lint: 0 errors, existing warnings only
• npm run build: pass

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