fix(web-ui): compose Observe panel through Workbench

[kachofugetsu09]

Summary

• register the Observe dashboard through workbench.panels.v2
• preserve the established monitor layout and plugin-owned Tailwind output
• make list, detail, and status mutations obey the active panel owner

Depends on kachofugetsu09/akashic-agent#520.

Validation

• 16 passed
• controlled browser E2E proves stale range/detail reads cannot replace current state
• controlled status E2E proves one mutation, disabled duplicate action, close abort, and no cross-row commit
• desktop pixel baseline: zero difference