Switching the main model never touches auxiliary slot pins (they're
independent, sticky per-task overrides). A user who switches main away
from a now-unpaid provider keeps paying 402s on every background aux call
until they manually reset those pins — silently, with no UI signal.
- /api/model/set scope:'main' now returns stale_aux: slots still pinned
to a provider different from the new main (additive field).
- Desktop Model Settings shows a switch-time notice after Apply AND a
persistent banner when any loaded aux slot mismatches the main provider,
both wired to the existing 'Reset all to main' action.
- Never auto-clears pins — a dedicated cheaper aux model is a legitimate
config; surface-and-offer instead of nuking.
- Fixes a stale pre-existing assertion in the panel test (main model now
renders via selectors, not a standalone label).
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