The settings UI previously required selecting a settings scope (User /
project) before entering a sub-page; the scope picker disappeared once
inside. This makes the scope switchable from within sub-pages.

- The scope segment of the sub-page breadcrumb (e.g. "User / AI / Agent
Configuration / Skills") is now a dropdown listing all scopes the
sub-page supports (filtered by the sub-page's file mask, intersected
across the sub-page stack). Selecting a scope re-renders the sub-page in
place — e.g. the Skills page swaps between global and project-local
skills.
- The sub-page stack is preserved across scope changes when every
sub-page in it is valid in the new scope; otherwise it falls back to the
regular navigation behavior.
- Sub-pages that only support one scope (Tool Permissions, Configure
Providers, Feature Flags) keep the static label.
- Entering a sub-page now clears the settings search bar so sub-pages
show all their items (settings deep links that prefill the search are
unaffected).

Closes AI-343

Release Notes:

- Improved the settings window: sub-pages such as Skills and
per-language settings now have a scope picker in the breadcrumb, so you
can switch between User and project settings without leaving the page.

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>