bug(ui): Anonymous public-chat history never restores after reload (session id lowercased on lookup) #3443

Summary

The empty "Start a Conversation" state. GET /api/public/history/<token>?session_id=<id> returns 200 with {"messages":[],"message_count":0}; the client sends the stored id with its case intact.

Category: functional · Severity: Critical · Validated: source

Context

Found by the click-through UI sweep of a local instance (dev @ c90c711ee, 1.0.0-rc1), fixture agents only.

Repro

1. Create a public link on an agent whose access policy has the email gate off.
2. Open /chat/<token> logged out (1440, light) and send one message.
3. Reload the page.

Expected

The visitor's conversation is restored after a reload ("Refresh keeps place").

Observed

The empty "Start a Conversation" state. GET /api/public/history/<token>?session_id=<id> returns 200 with {"messages":[],"message_count":0}; the client sends the stored id with its case intact.

Validation

src/backend/db/public_chat.py:146 — the lookup compares session_identifier == session_identifier.lower() unconditionally, while the create path (line 84) lowercases only when identifier_type == 'email', so a mixed-case anonymous token never matches its own row.

Acceptance Criteria

• The repro above gives the Expected result at 1440 and 768, in light and dark.
• A regression test pins the behaviour (unit or Playwright, whichever tier can observe it).
• No new raw-palette classes or hand-rolled primitives are introduced by the fix (docs/memory/design-system-contract.md).

Technical Notes

Evidence screenshots (kept on the machine that ran the sweep; the evidence folder is gitignored because the repository is public):

• docs/testing/ui-sweep/evidence/2026-10-09-all/d4a6e8df-1.png

Found by /ui-sweep --all @ c90c711