feat: session rename + graph chrome polish (popovers dismiss, no-shift no-match, coverage reseat)

Rename (owner ask): clicking the header title opens edit mode with the
whole title selected — typing replaces, arrows move inside. Enter or a
click away commits (PUT /api/chats/:id); Escape and an empty commit
discard. renameChat joins the ChatStore port; like saveSettings it is
NOT conversation activity — updatedAt stays put so sidebar order never
churns on a rename. The sidebar patch is instant: the header dispatches
lh:chat-renamed and SessionsSidebar folds it into its snapshot (the 2.5s
poll is only a backstop). Sidebar right-click rename is DELIBERATELY
deferred: it needs a real context-menu system, which gets its own pass
later (owner decision, 2026-09-17).

Graph chrome:
- The extraction-coverage button moves between search and import; the
  gear keeps the far-right corner. Coverage still renders on an EMPTY
  graph (that is exactly when it matters).
- The no-match search note no longer shifts its siblings: the chrome
  row is items-start (every button is 44px tall, so top alignment pins
  the row while the search container grows downward).
- Popovers dismiss properly: pointerdown anywhere outside the control's
  root (button + panel) closes it, and so does Escape. Two panels can
  never be open at once — the sibling chrome buttons are outside the open
  root, so pointerdown closes the first before the click opens the
  second. New shared hook components/popover-dismiss.ts, wired into
  both the settings gear and the coverage control.

Gates: typecheck, eslint, 498/498 (+2 rename store tests), build.
Live-verified: select-all/focus/Enter/Escape/empty/blur round-trip,
updatedAt untouched, sidebar instant patch; chrome order + frozen
positions under a no-match query; popover close via canvas, search
input, Esc, mutual exclusion, toggle intact.

components/popover-dismiss.ts

@@ -0,0 +1,32 @@
+import { useEffect, type RefObject } from "react";
+
+/**
+ * Close-on-dismissal for floating panels (graph chrome, 2026-09-17):
+ * any pointerdown landing OUTSIDE the control's root closes it; Escape
+ * closes it too. The root contains both the toggle button and the panel,
+ * so clicks on the control's own button stay "inside" — its toggle keeps
+ * working — while a click on a sibling chrome button counts as outside:
+ * pointerdown closes the open panel first, then the sibling's click opens
+ * its own. Two dropdowns can therefore never be open at once.
+ */
+export function usePopoverDismiss(
+	rootRef: RefObject<HTMLElement | null>,
+	open: boolean,
+	close: () => void,
+): void {
+	useEffect(() => {
+		if (!open) return;
+		const onPointerDown = (e: PointerEvent) => {
+			if (rootRef.current && !rootRef.current.contains(e.target as Node)) close();
+		};
+		const onKeyDown = (e: KeyboardEvent) => {
+			if (e.key === "Escape") close();
+		};
+		document.addEventListener("pointerdown", onPointerDown);
+		document.addEventListener("keydown", onKeyDown);
+		return () => {
+			document.removeEventListener("pointerdown", onPointerDown);
+			document.removeEventListener("keydown", onKeyDown);
+		};
+	}, [open, close, rootRef]);
+}