[FEATURE]: persist model selection per agent in desktop app (like TUI) #28110

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently the desktop app saves model selection per session. When switching agents within a session, the model stays the same rather than restoring the last model used for that agent. The TUI handles this better by storing model per agent name.

Proposed approach:

• Add a "Persist model per agent" setting (opt-in, default off)
• Add a "Persist model across sessions" setting (default on, preserves current behavior)
• When per-agent is enabled, model selection is stored keyed by agent name
• When switching agents, the last used model for that agent is restored
  This keeps the current behavior as default while giving users the flexibility to opt into the per-agent behavior that the TUI provides.