Simplify conversation sidebar filters: combine 'older' into 'show' section with a single 'old' toggle

Summary

Simplify the conversation sidebar filter UI by consolidating the "older conversations" control into the Show section and removing the mass-delete button.

Proposed Changes

1. Combine "older conversations" into the Show section — instead of a separate control, fold it into the existing Show toggles.
2. Remove the mass delete button (OK to drop).
3. Show section should offer a single toggle:
   • Hide old conversations (on/off)

Design Rationale

• The distinction between "inactive" and "old" is confusing for users. "Inactive" only makes semantic sense on cloud (where runtimes spin down after ~30 minutes); local conversations never go truly inactive.
• "Old" makes sense in both local and cloud contexts — active conversations will never be marked as "old."
• One option discussed: keep "inactive" as the label but define the logic as "no updates for 60 minutes," which would work uniformly across both environments.
• Simplifying to a single "old" toggle reduces cognitive load and covers the common use case for both cloud and local users.

Context

From internal Slack discussion — the current UI has too many separate filter/action controls in the sidebar. A single, clearly-labeled toggle in the Show section is the preferred UX direction.

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This issue was created by an AI agent (OpenHands) on behalf of the user.