fix: show warnings when no LLM provider configured for prompt actions

[SeoFood]

Summary

• Show a MessageBox warning when executing a prompt action with no LLM provider available, instead of silently doing nothing (fixes Prompt actions require manual LLM provider configuration per action #8)
• Add per-action provider override dropdown to the prompt editor UI (the data model already supported it but the UI was missing)
• Make the "(Default)" provider label context-aware, showing which provider it resolves to or "(Default - none configured)"
• Show overlay feedback in dictation mode when a profile's prompt action is skipped due to missing provider

Test plan

• Create a prompt action with no provider override and no global default - executing should show a warning MessageBox
• Verify the default provider dropdown shows the resolved provider name (e.g. "(Default - OpenAI / gpt-4o-mini)")
• Verify the prompt editor now has a "Provider" dropdown to set per-action overrides
• Verify per-action provider override persists after save and reload