feat: consolidate conversation filters with hide toggles

[sjathin]

• A human has tested these changes.

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Why

The conversation sidebar has a confusing UX: an "older conversations" collapsible group that conflates "inactive" and "old," plus a mass delete button placed inline with filter toggles. This makes the filter menu cluttered and the semantics unclear across local vs cloud environments.

Summary

• Replace the "older conversations" collapsible group and mass delete button with two on/off toggle filters ("Hide inactive conversations" and "Hide old conversations") in the Show section of the filter menu
• Both toggles default to off (all conversations shown by default) and use a 60-minute threshold
• Remove the mass delete button as part of this cleanup
• Conversations are now rendered as a single sorted list (by the user's chosen sort field — "Updated" or "Created", descending) instead of being split into recent/older groups

Issue Number

Closes #675

How to Test

1. Run npm install && npm run dev:docker (or npm run dev:dangerously-dockerless)
2. Open http://localhost:8000/
3. Create or have conversations older than 60 minutes
4. Click the filter icon (funnel) in the sidebar conversation list header
5. In the "Show" section, verify:
   • "Hide inactive conversations" and "Hide old conversations" toggles appear with Eye/EyeOff icons
   • Clicking either toggle hides conversations older than 60 minutes
   • Clicking again (label flips to "Show...") restores them
6. Verify the "Older conversations" section and "Delete all" button are gone
7. Verify other filter options (Organize, Sort by, Metadata) still work

Video/Screenshots

Screen.Recording.2026-05-25.at.16.01.07.mov

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

Both "Hide inactive" and "Hide old" currently use the same 60-minute time-based threshold, as specified in the issue. The issue rationale explains that "inactive" (no updates for 60+ minutes) is a universal definition that works for both local and cloud — on cloud, runtimes spin down after 30m, but the 60m threshold covers both environments. A follow-up could differentiate them (e.g., making "inactive" execution-status-based) if the team decides they should diverge.

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