[FEATURE] Mods: run outside a chat session (app-level host)

Problem

A mod lives inside a session: its panes and hooks exist only while some chat is running it. A dashboard that watches all chats therefore needs a chat to host it. The workaround is an empty "Dashboard" chat that is never typed in, which clutters the sidebar and depends on that chat staying open.

Proposal

An app-level host for mods, such as a sidebar or window pane in the desktop app, that loads a plugin's UI and background hooks ($.clock, $.fs, $.process, $.session.send) with no conversation and no model attached. Session-bound APIs would be unavailable there or would target a chosen session.

Why we want it

A cross-chat view (which chats are running, blocked or idle, what each subagent is doing, how full each context window is) belongs to the workspace, not to any single chat. Running it at app level would make it always available, with no token cost and no throwaway chat.

Related: #91870 (Mods), #24537 (agent dashboard).