Persist selected board and auto-select most recent image across browser sessions (#92)

* Persist selectedBoardId across browser sessions

Co-authored-by: lstein <111189+lstein@users.noreply.github.com>

* fix(frontend): make appStarted listener async so image auto-selection works on startup

Co-authored-by: lstein <111189+lstein@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lstein <111189+lstein@users.noreply.github.com>

Author: Copilot