fix(desktop): show default profile in switcher

[tt-a1i]

Fixes #48583

Summary

• split the desktop profile rail's All profiles aggregate from the default/root profile entry
• show the default profile name as a compact visible pill while preserving the existing named-profile squares
• add regression coverage for default visibility and mutually exclusive All/default active states

Test plan

• npm run test:ui --workspace apps/desktop -- src/app/chat/sidebar/profile-switcher.test.tsx
• npm run typecheck --workspace apps/desktop
• cd apps/desktop && npx eslint src/app/chat/sidebar/profile-switcher.tsx src/app/chat/sidebar/profile-switcher.test.tsx
• npx prettier --check apps/desktop/src/app/chat/sidebar/profile-switcher.tsx apps/desktop/src/app/chat/sidebar/profile-switcher.test.tsx
• git diff --check

Note: full desktop lint/test were also sampled, but currently fail on existing unrelated files/environment setup (for example missing jsdom localStorage, node:test .cjs suites under Vitest, and unrelated lint errors).