docs: update stale tabs.js→swarm.js/call.js references

After the SW4RM and Y3R C4LL extractions from tabs.js (PRs merged
earlier), several docs still pointed at `assets/tabs.js::fn`. Update
them to the authoritative module:

- dashboard.md: renderContainers, buildAgentTree → swarm.js
                renderApprovals → call.js
                renderPeerHives → swarm.js
- shape.md: generic "tabs.js render function" → "domain module"
- conventions.md: clarify async-form listener lives in tabs.js (dashboard)
                  AND app.js (per-agent UI)

docs/conventions.md

@@ -406,7 +406,7 @@ the relevant handler (`agent_server.rs`, `mcp.rs`, or `dashboard.rs`).
 ## Async forms
 
 Dashboard + per-agent mutating forms carry `data-async`; a delegated
-`submit` listener in `assets/tabs.js` intercepts, shows a spinner,
+`submit` listener in `assets/tabs.js` (+ `assets/app.js` for the per-agent UI) intercepts, shows a spinner,
 POSTs `application/x-www-form-urlencoded` (axum's `Form` extractor
 rejects multipart), calls `refreshState()` on success. New mutating
 forms should add `data-async` and optionally `data-confirm` (for a