feat(tui): add manual update dialog - #47364

Summary

• Add /update and Update OpenCode in the command palette with a centered check, confirm, install, and restart dialog.
• Open known updates directly from the Home notification without checking again; keep only the local update entry point and the remote-server notice on Home.
• Keep manual update operations separate from automatic notification history, and reuse the existing release-channel lookup and installation methods.
• Exit normally after an update; the user reopens OpenCode, and the existing startup path handles managed-server version mismatches.
• Restore the previous dialog's shimmer styling, share its masked-text rendering with the existing fade animation, and remove temporary debug previews and obsolete inline Home update states.

Review cleanup

• Preserve an already-installed version when opening a stale Home notification.
• Cancel in-flight checks when their dialog closes so late results cannot overwrite an installation.
• Remove the self-relaunch process wrapper and leave shutdown/startup handling unchanged.
• Remove duplicate success states, unused notification APIs, redundant Home wrappers, and obsolete multiline Home-message rendering.

Validation

• Formatting and whitespace checks passed.
• No tests added or run, as requested.
• Three delegated reviews covered reuse, code quality, and efficiency. Findings were addressed; follow-up review found no remaining actionable issues.