feat(public-chat): chat history dropdown for logged-in users (#587)

[vybe]

Summary

• Logged-in Trinity users visiting a public agent link now see a History dropdown in the header listing their previous chat sessions with that agent (last 20, most-recent first)
• Past sessions load in read-only mode with an amber banner and "Return to current chat" — no false continuity with the live agent context window
• Not-logged-in users: no change to existing UI

Changes

Backend (src/backend/routers/public.py):

• GET /api/public/sessions/{token} — JWT-authenticated; returns caller's sessions for the agent behind this public link (bypasses get_authorized_agent which would 403 non-owner visitors)
• GET /api/public/sessions/{token}/{session_id} — JWT-authenticated; session detail with ownership validation

Frontend:

• src/frontend/src/components/chat/ChatHistoryDropdown.vue — new dropdown component (click-outside aware, date formatting, error/loading/empty states)
• src/frontend/src/views/PublicChat.vue — imports useAuthStore, shows dropdown for authenticated users, read-only mode with banner

Tests (tests/test_public_chat_history.py): auth requirements, 404 on invalid tokens, limit param validation

Test Plan

• Tests pass: pytest tests/test_public_chat_history.py -v (5 pass, 1 expected skip)
• Not logged in: History button absent, no visible change
• Logged in, no sessions: History dropdown shows "No previous sessions found"
• Logged in, sessions exist: dropdown lists sessions with date/count/preview
• Click session: messages load, amber read-only banner shows, input hidden
• "Return to current chat": exits read-only, reloads live session

Fixes #587

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