#27103 Fix running tray icon and composer spacing

What changed

Leo said:

“Also why is this? the running section. I NEVER SAID TO PUT THE SPINNER HERE. But why is this, like, upwards, man? Like why is it even spaced like this? Why does it look like this?”

Leo repeated today: “you still havent merged this weird pad situation in the bottom running bar”.

This PR makes the two requested tray changes:

• Restores the fixed agent.session icon in the running tray header. The spinner remains on each agent's working row.
• Removes the tray's bottom margin so it sits flush with the composer stack.
• Adds the one-agent-two-commands gallery state (1 subagent running · 2 commands) for light and dark before/after capture.

The tray's outer width and side insets come later with #26637; this PR contains no width or inset work.

Validation

• Node 24: bun test test/subagent-tray.test.tsx test/subagent-tray-layout.test.ts test/subagent-tray-motion.test.tsx (42 passed)
• Local Node 24 scripts/web-lint.sh is being rerun on the decoupled five-file head.
• CI: one combined check at the new head is requested below.

Risk: normal.

Agent session: agent session