Fix channel header shared blur layering

[thomaspblock]

Summary

• add a single shared channel header blur/backdrop across the channel + split auxiliary columns
• render channel and auxiliary header chrome transparently above the shared blur so header text/actions remain crisp
• keep the split divider/resize affordance above the blur layer and remove split-pane enter motion stacking that clipped the backdrop
• add smoke coverage for the shared backdrop span and layering

Testing

• pnpm -C desktop typecheck
• pnpm -C desktop check
• pnpm -C desktop run build
• pnpm -C desktop exec playwright test tests/e2e/channel-shared-header-backdrop.spec.ts --project=smoke

Notes

• Tested locally against staging with BUZZ_SHARE_IDENTITY=1 just staging.

[thomaspblock]

Screenshots

Channel header shared backdrop

Single shared blur element spans the channel header area, with header text/actions above it and message content scrolling behind it.

Split thread panel shared backdrop

Latest layering screenshot: the same shared blur bleeds across the main channel and right thread panel while the divider/header stay above the blur.

[wesbillman]

@thomaspblock in those screenshots, how do we close a thread sidepanel after it's open? I don't see a close button there anymore

[thomaspblock]

it added some previous screenshots for some reason.

I've asked it to update with the latest.

Here is the one from my machine.