feat: move Settings to the sidebar footer and add settings-specific tabs with back navigation

[Fanzzzd]

The current settings page works, but the information architecture may not scale well once more settings categories are added.

Requested change

• Put the Settings entry in the lower-left sidebar/footer area.
• When entering settings, replace the left sidebar content with settings-specific navigation tabs/sections.
• Add a clear Back action to return to the normal project/thread sidebar.

Why

This would make the app feel more structured once settings grows beyond a single long page, and it matches the expectation that global app settings live in the sidebar footer rather than in the main project navigation flow.

Suggested acceptance criteria

• Settings is reachable from the sidebar footer.
• Entering settings swaps the normal left sidebar content for settings navigation.
• Users can move between settings sections without scrolling one long page.
• A visible back action restores the normal sidebar/project view.

Relevant code context

• apps/web/src/components/Sidebar.tsx
• apps/web/src/routes/_chat.settings.tsx

[shivamhwp]

i think it's fixed