[idea] user-configurable state icons, as part of theming

[vukvukovich]

idea / problem

State indicators are getting distinct per-state shapes (#2260, pending release), but the glyphs themselves stay compiled in. Themes control every color in the UI, not these glyphs. I customized state icons via pane-metadata tokens and custom agent rows; that covers expanded rows only - the collapsed rail and toasts keep the built-ins, so the result is inconsistent across surfaces (and as #824 noted, plugins cannot restyle the UI).

requested change

A config override for the per-state glyph mapping that #2260 unifies: each state takes a single glyph or a list of animation frames (frame lists would subsume the opt-in animated style from #2534, with the reduce-motion concerns from #1316 applying). Defaults unchanged. Theming feels like the natural home since the palette already owns the state colors, but [ui] would serve equally. The per-agent-kind icons requested in #2358 could share the same "icon overrides" home.

why you want this

Setups differ in font capability and taste: with a nerd font I want richer state sets (per-state robot variants, brand glyphs), while minimal setups may want plain ASCII. One compiled-in set cannot serve both, and the per-surface workaround leaves the sidebar inconsistent between expanded and collapsed views.