Fix case-sensitive fleet and run spec intersection

[r4victor]

A fleet spec and a run spec were intersected case-sensitively, so a fleet declaring gpu: mi300x and a run requesting gpu: MI300X produced an empty GPU name list that matches nothing. The run then got no backend offers at all - only offers from instances that already existed in the fleet - while dstack offer, which ignores fleets, kept returning them.

• Intersect GPU names, regions, availability zones, and instance types case-insensitively when combining fleet and run specs, keeping the spelling from the fleet spec.
• Combine compute_capability with max rather than min, since it is a lower bound and the stricter of the two must win.
• Compare availability zones case-insensitively when filtering backend offers and existing instances, as regions and instance types already were.

Fixes #4214.

AI assistance: written with Claude Code.