fix(desktop): limit channel composer blur to input

[tlongwell-block]

Summary

• remove the full-width backdrop blur from the main channel composer overlay
• retain blur on the inner message composer card
• assert the overlay stays transparent while the composer remains blurred

Root cause

The main channel overlay acquired background and backdrop-blur classes in #1698, while the thread overlay stayed transparent. Since the shared MessageComposer already blurs its own card, the main channel rendered an extra full-width blurry strip around the composer.

Test plan

• pnpm --dir desktop typecheck
• pnpm --dir desktop exec biome check src/features/channels/ui/ChannelPane.tsx tests/e2e/channels.spec.ts
• pnpm --dir desktop build
• cd desktop && pnpm exec playwright test ./tests/e2e/channels.spec.ts --grep "channel date divider keeps the date sticky"
• just desktop-screenshot --name compose-blur-fixed --route /channels/general --active-channel general (visually inspected)
• pre-push hooks: desktop check/tests, mobile tests, Rust tests, desktop Tauri tests

[tlongwell-block]

Max review at current head 54e0265df: no blocking findings. The main-channel layering now matches MessageThreadPanel exactly: transparent outer overlay, shared blurred MessageComposer form, and opaque/non-blurred activity + typing row. The computed-style regression assertions cover all three layers. Independently verified pnpm build and the focused channels.spec.ts smoke test (1 passed). I could not submit a formal GitHub approval because the shared CLI identity owns this PR.