[BUG] "Group by: Folder" groups nothing after desktop profile reset — even brand-new sessions with known cwd land in "Other"; creating a manual group silently flips Group by back to Custom groups

Description
Since a Claude Desktop profile reset (2026-08-22, Reset-AppxPackage wiped LocalCache), the session→folder association never reaches the server for any session on this account — including brand-new sessions created after the reset whose working directory is fully known locally. With the sidebar set to "Group by: Folder", zero folder groups form: every session (old and new) pools into "Other".

This looks like the same root cause suspected in #89781 (device/environment identity changed, server-side binding lost), but with a stronger observable: there, pre-reinstall sessions were orphaned; here, even fresh sessions in fresh folders never bind, so "Group by: Folder" is entirely non-functional for the account.

Environment

• OS: Windows 11 Home 10.0.26200
• Claude Code CLI: 2.1.241
• Claude Desktop (Windows), current as of 2026-08-29
• History: full desktop profile reset on 2026-08-22 (Reset-AppxPackage); transcripts in ~/.claude/projects intact

Steps to reproduce

1. On claude.ai/code (or the desktop app), set sidebar filter → "Group by" → "Folder".
2. Create a brand-new session from the Desktop app inside a project folder (e.g. Desktop\<project-folder>).
3. The local transcript is correctly written under ~/.claude/projects/<encoded-cwd-path>/<uuid>.jsonl — the client clearly knows the folder.
4. Observe the sidebar: no folder group is ever created; the session stays in "Other"/"Ungrouped" indefinitely (verified >20 minutes and across reloads). Repeated with a second new session in a different folder — same result.

Expected
A session created inside a folder appears under a sidebar group named after that folder (the behavior before the profile reset).

Actual
"Group by: Folder" renders a single "Other" bucket containing every session. No folder group forms for any session, new or old.

Separate but compounding UX bug: silent mode flip
Creating (or moving a session into) a manual custom group silently switches "Group by" back to "Custom groups" with no notice. Observed live twice on this account. Combined with the binding defect, the user experience is: enable Folder grouping → nothing groups → user creates a group manually as a workaround → the mode silently reverts → user concludes grouping is randomly broken. If the flip is intended, it should at least be announced (toast/confirm).

What was checked locally

• Transcripts intact and written live under the correct cwd-derived project directories.
• No local file controls sidebar grouping (settings.json, project registry inspected) — the state is server-side, so no local remediation exists.
• In "Custom groups" mode, an environment already bound to a group still auto-joins new sessions of that environment — so the group-membership pipeline works; it is specifically the folder identity that never arrives.

Impact
Organizational only, but total: automatic folder grouping is unusable for the account, and the silent mode flip makes the failure look nondeterministic to users.

Related: #89781 (pre-reinstall sessions stuck in "Other"; suspects machineID change — likely same root), #89134.