allozaur/feat/chat slash commands (#26716)
* base : slash-command/misc foundation - model icon and focus-selector constants
* feat : slash-command picker and command parsing helpers
* refactor : wire command and @-mention pickers into the chat form
* ui : improve model selector keyboard navigation and load/dismiss
* feat: Unify markdown/raw-text rendering under one setting with migration
* fix: Misc fixes - tool-call subtitle, assistant wrap, progress guards
* feat: Clamp and style numeric settings inputs from registry bounds
