Archived sessions render inline in the sidebar project group instead of a separate Archived list

## Summary

Sessions correctly marked `isArchived: true` are still listed in the main sidebar, inline within their project group. On an account with a few months of work this means ~70 archived rows are stacked under a handful of active ones, and there appears to be no way to hide them.

Archived rows are visually distinguished (dimmed, with an archive-box icon), but they are not separated out — they occupy the same list as active sessions.

## Environment

- Claude desktop app: 1.40609.1
- Bundled Claude Code: 2.1.255 (CLI on PATH: 2.1.80)
- macOS (Darwin 25.6.0), Apple Silicon
- `sidebarMode`: `epitaxy`

## Steps to reproduce

1. Accumulate sessions in a single project group over time.
2. Let them archive — either manually or via auto-archive.
3. Expand that project group in the sidebar.

## Expected

Archived sessions move out of the main list into the separate Archived view. The app's own `archive_session` tooling documents this behavior:

> Archiving stops the session's process and (by default) cleans up its worktree; the session can still be reopened later from the **Archived list**.

## Actual

Archived sessions remain in the project group in the main sidebar, dimmed but present. In my case the group renders ~70 archived rows above/below the ~6 active ones, which makes the active sessions hard to pick out.

## Evidence

The local session records are correct — the flag is set, it just isn't being honored by the sidebar. From the local session store:

```
archived: 70 not archived: 10 (80 records, single workspace)
```

Every archived record carries `isArchived: true`. Notably, none carry an `archivedAt` timestamp or a `lifecycleState` field, so the flag is the only signal available to a consumer.

Auto-archive is enabled and working correctly — it is what produced the 70:

```json
"ccAutoArchiveOnPrClose": true,
"ccAutoArchiveInactiveDays": 30
```

So this is not an archiving failure. Archiving succeeds; the sidebar just doesn't act on the result.

## What I ruled out

- **A user setting.** No show/hide-archived preference exists in `~/.claude/settings.json`, `settings.local.json`, or the desktop `preferences` block. The only sidebar preference is `sidebarMode` (`chat`/`code`/`task`/`epitaxy`), which controls layout, not filtering.
- **A string in the local bundle.** Searching the app bundle turns up `Unarchive` but no "Show archived" / "Hide archived" affordance.
- **Local renderer code.** The main-window renderer in the bundle is a ~323KB shell; the sidebar UI is served remotely, so there is nothing installed locally that could be adjusted as a workaround.

## Impact

The only workaround is collapsing the whole project group, which hides the active sessions too. For anyone who works in one repo for a sustained period, the sidebar becomes progressively less usable — the ratio here is roughly 10:1 archived to active.

## Suggested fix

Either move archived sessions into the separate Archived list the tooling already references, or add a show/hide-archived control to the sidebar group header.

No repository file, test, or entry point is identified; the report says the sidebar UI is served remotely and the local renderer is only a shell. First confirm whether the sidebar implementation is in this repository and locate its session-list filtering and Archived-list entry points. Done means archived sessions no longer appear in project groups and remain accessible from the separate Archived view, with coverage for records carrying only isArchived.

Scritto dal modello di indicizzazione a partire dal testo della issue.