feat: Enhanced custom model profiles with multiple models per profile

[emsanakhchivan]

Summary

• Support multiple models per custom profile (e.g., multiple models under one OpenRouter API)
• Custom models now appear individually in model selector dropdown with profile name as subtitle
• Added visibility toggle for custom models in Settings > Models section
• Fixed IPC transport to use activeConfigAtom for proper profile/model selection
• Added search filtering for custom models in Settings
• Removed redundant "Use" button from profile management (models are selected directly from dropdown)

Changes

• Updated ModelProfile type to include models: CustomModelConfig[] array
• Added activeCustomModelIdAtom to track selected model within a profile
• Added migration logic for old profile structure
• Updated AgentModelSelector to show individual custom models with profile subtitle
• Added hiddenModelIds prop to filter hidden models from dropdown
• Fixed ipc-chat-transport.ts to use activeConfigAtom instead of legacy customClaudeConfigAtom
• Added Custom Models subsection in Settings > Models with visibility toggles
• Added search filtering for custom models in Settings

Test plan

• Create a custom profile with multiple models
• Verify each model appears separately in the model dropdown
• Verify profile name shows as subtitle under each custom model
• Toggle custom model visibility in Settings and verify dropdown updates
• Search in Settings and verify custom models are filtered
• Edit/remove profiles and verify changes persist