Sidebar: always show Sessions, collapse (#498)

Browse by default, add an Admin link for admins Switching to Files, Crons, or any non-chat view blanked the sidebar's Sessions list, hiding the most-used navigation exactly when you were elsewhere. The Sessions header and list now render in the sidebar on every view. The Browse nav group starts collapsed by default (a stored preference still wins once toggled), and users with the admin permission get an Admin link at the bottom of Browse pointing at the admin home.

<!-- codesmith:footer -->
---
<a href="https://app.blacksmith.sh/yc-software/codesmith/qm/pr/453"><picture><source media="(prefers-color-scheme: dark)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"><source media="(prefers-color-scheme: light)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-light-v2.svg"><img alt="View with [code]smith" src="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"></picture></a> <a href="https://backend.blacksmith.sh/track/enable-autofix?expires=1789245659&installation_model_id=19911&pr_number=453&repository=yc-software%2Fqm&return_to=https%3A%2F%2Fgithub.com%2Fyc-software%2Fqm%2Fpull%2F453&signature=65cf5399895a8ad2e24a57ddb0fc077030538268d47784698128885c898d489e"><picture><source media="(prefers-color-scheme: dark)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"><source media="(prefers-color-scheme: light)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-light.svg"><img alt="Autofix with [code]smith" src="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"></picture></a>
<sup>Need help on this PR? Tag <code>@codesmith-bot</code> with what you need. Autofix is disabled.</sup>

<!-- codesmith:autofix:disabled -->
<!-- /codesmith:footer -->