feat: support multiple Honcho instances

[offendingcommit]

Closes #2.

Summary

• Replace the single-config localStorage model with a named-instance store ({ instances, activeId }); existing users are migrated transparently on first load.
• Settings page becomes an instances manager — list, activate, edit, delete, add new — with the existing connection form reused for create + edit flows.
• Sidebar shows the active instance name + URL with a click-through switcher dropdown when more than one instance is configured.
• Switching or deleting an instance clears the TanStack Query cache so data from another deployment never bleeds into the active view.

Test plan

• make check (lint + typecheck + test) passes
• Fresh load with no config shows the create form and "Connect to your self-hosted Honcho instance" copy
• Existing users with the old openconcho:config key get auto-migrated to a single "Default" instance
• Adding a second instance lights up the sidebar switcher; clicking switches and refetches
• Editing an instance updates name / baseUrl / token without losing the active state
• Deleting the active instance promotes another (or returns to first-run state if it was the last)