Persist session ID in sessionStorage for tab identity

[cstns]

Description

• tabSessionId added to state (separate from the agent stores' chat sessionId)
• Persisted to sessionStorage so it survives page refreshes within the same tab
• Auto-generated on first hydration if not present
• Not consumed by anything yet. The MCP button (MCP header button: opt-in browser session exposure for third-party agents #8159) will be the first consumer.

The agent stores, expert store, and post-message service are all untouched. Chat session IDs remain as they are.

Addendum: reuse account-auth session ID instead of a new tabSessionId

The original approach added a new tabSessionId to the context store. After further discussion, we realized the account-auth.js store already has a per-tab sessionId (generated via getSessionId(), used by the fe-team MQTT connection for broker client uniqueness). The only problem was that it was stored in-memory, so it died on page refresh.

By persisting it to sessionStorage, it becomes exactly what tabSessionId was supposed to be: per-tab, stable across refreshes, unique across tabs. This also means we can reuse the existing fe-team MQTT connection for presence topics on the backend, so there's no need for a separate MQTT connection or a new client identity for MCP presence. The tabSessionId addition to the context store was reverted.

The change simplifies to: persist the existing account-auth session ID in sessionStorage (split the persist config into an array with localStorage for redirectUrlAfterLogin and sessionStorage for sessionId).

Related Issue(s)

closes #8157

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.19%. Comparing base (9515085) to head (5d62084).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

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  Coverage   76.19%   76.19%           
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  Files         441      441           
  Lines       23619    23619           
  Branches     6289     6289           
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  Hits        17996    17996           
  Misses       5623     5623           

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[cstns]

see #8157 (comment)