feat: dialog for per-model context size, vision and prices

Field conversion is separated from the widget so the part with the edge
cases is testable without a running Qt application. Vision is tri-state
through a binary checkbox: an unchecked box with an unknown prefill stays
unknown rather than writing False, which would shadow a provider-level
vision=True through models.resolve's pick().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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