fix(desktop): retain terminal focus on viewport click

[tlongwell-block]

Summary

• preserve terminal ownership when the active terminal viewport is clicked
• prevent the canvas mousedown default before it can blur the offscreen input, then defensively refocus it
• leave tab-bar mouse semantics and concealed/Buzz-owner focus untouched

Regression proof

At base d8a4d63a8, the new browser test failed after the viewport click with Terminal input inactive. With this commit, the full terminal-wheel.spec.ts passes all 3 arms:

• existing wheel delivery
• active viewport click retains document.activeElement and delivers FOCUS_KEYSTROKE to terminal_input
• concealed/Buzz-owner viewport click does not focus the terminal textarea

Verification

• pnpm check
• pnpm typecheck
• pnpm test — 3,992/3,992
• pnpm build:e2e && pnpm exec playwright test tests/e2e/terminal-wheel.spec.ts --project=smoke — 3/3
• pre-push: branch-skew, desktop-check, desktop-test, mobile-test, desktop-tauri-checks, rust-tests all green
• post-push: remote/local SHA match; git diff --quiet HEAD passes