feat: replace sidebar row icon with prefix and suffix slots

AGENTS.md

@@ -64,7 +64,7 @@ GPUI is a UI framework which also provides primitives for state and concurrency
 ## Rendering and state
 
 - `Render` builds an element tree from entity state; `RenderOnce` (with `#[derive(IntoElement)]`) is for value-like components whose `render` takes `self` and `&mut App`.
-- Keep `render` declarative; use `.when(...)` / `.when_some(...)` for small conditional refinements.
+- Keep `render` declarative: build elements as one chained expression, using `.when(...)` / `.when_some(...)` / `.map(...)` for conditional refinements instead of `let mut` element variables reassigned under `if` / `if let`.
 - Controlled components: the owner passes the current value, callbacks report intent, the owner updates state and calls `cx.notify()`. No second source of truth.
 - Call `cx.notify()` after a mutation that changes rendering; notify the narrowest owning entity. Never notify from `render`.
 - `render` is side-effect free: no `cx.notify()`, no `entity.update(...)`, no writing derived text or state back into other entities during render. A write there forces another frame, and a write that emits an event re-enters the event handler — an unbounded redraw loop. Reconcile state in event handlers instead, with silent setters (`set_value_silent`-style) so a control's own write does not re-enter the events it mirrors.