feat(scheduler): separate image-generation AUTO chain with UI toggle

Image models previously could not be scheduled through a priority
chain: the chat AUTO chain explicitly skips image-kind slots, and
AUTO image requests fell back to unordered registry discovery.

- config: add auto_image rules (auto_image yaml / image_rules json);
  legacy auto rules keep their meaning as the chat chain
- core: buildAutoImageChain mirrors buildAutoChain with inverted kind
  filter (image-only); SaveAutoImageRules + AutoImageRules/AutoImageChain
- scheduler: ChainImage walks the chain tier-by-tier with round-robin
  and preference ordering, skipping cooling slots
- gateway: handleImage AUTO now runs down AutoImageChain when one is
  configured (falls back to legacy discovery otherwise) and records
  the actual served model; handleAutoAPI GET returns image_rules and
  PUT accepts image_rules independently of rules
- webui: priority page gains a chat/image toggle editing two
  independent lane sets; add-slot picker filters by active kind;
  persistAuto writes only the active chain's field

.gitignore

@@ -20,3 +20,4 @@ cmd/gui/*.log
 
 # local ops scripts (machine-specific, not for sharing)
 scripts/
+cmd/gui/dist/