fix(tui): move background task count beside turn timer

[kingsword09]

Move the active background task count from the metadata statusline to the turn timer row above the input, for example [ 🕛 12s ] · 1 in background. Refresh this row when runtime projections change so the count stays current while idle and disappears when no background tasks remain.

Validation:

• bun run typecheck
• bun run build:tui
• 26 related tests passed (turn status, footer bar, status line, runtime polling, and turn work tracking).
• bun scripts/smoke-tui-features.ts
• PTY checks confirmed placement at 110 and 50 columns, count updates after task completion, and removal when the count reaches zero.

[pkg-pr-new]

npx https://pkg.pr.new/zcode-app-cli@c9fdf49

commit: c9fdf49