fix(desktop): stabilize session switching layout

[Colafornia]

Summary

• Keep live usage and permission controls mounted while switching sessions, so async reads do not reflow the composer footer.
• Reserve the chat scrollbar gutter with stable both-edges, preventing the composer from shifting when the transcript scrollbar appears or disappears.
• Keep the Permission icon visually stable while its action is temporarily disabled during boundary reads.
• Scope usage reads to the active session, model, and provider; show pending state instead of stale usage.

Before / After

State	Video
Before	Paste video here
After	Paste video here

Verification

• npm run build passed.
• npm run lint passed.
• npm run format:check passed.
• npm --workspace @maka/desktop run build-storybook passed.
• Full npm test was attempted; several environment-dependent suites failed because the local runner cannot open required sockets and file watchers (listen EPERM, EMFILE).
• Storybook smoke was attempted with elevated permissions; all but one unrelated existing model-picker assertion passed.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex contributed to the implementation, tests, and review.

Checklist

• Tests cover the change and fail without it
• Lint, format, build, and affected checks were run locally

Does this PR entail a change in behavior?

• Yes - the session-switch footer remains stable while data resolves
• No

[Colafornia]

Closing this PR because most of its changes duplicate the fixes already landed in #5683, while the remaining changes address issues that could not be reproduced.