Two composer fixes:
- **Paste/input lag** — `flushEditorToDraft` serializes the whole editor
(`composerPlainText` is O(n)); running it on every event during a burst
(holding a key, or holding Cmd+V into a growing editor) was O(n²). Coalesce
the input/paste path to one flush per animation frame. Lossless: the
contentEditable DOM is the source of truth and submit + the compositionend /
keydown paths re-read it synchronously (those stay immediate).
- **Detached-composer dock glow** — was `fixed inset-x-0` (full viewport, spilled
under the sessions sidebar). Switched to `absolute inset-x-0`, so it anchors to
the chat-column root the docked composer centers in — the glow now spans only
the thread area, matching the actual dock target.
Verified: typecheck clean, 0 lint errors, composer DOM repro tests pass.
33d91029b2