fix(web): make the re-auth banner a centered popup, not a full-width top bar

Unrelated to #2108's diagnostics work, folded in at the maintainer's request
rather than tracked separately.

The sticky full-bleed bar cost the whole view a band of vertical space for what
is a notification about one server, and it pushed down the monitoring sidebar
that an OAuth failure now opens. Every corner is spoken for — the toast stack
owns bottom-right, the right edge is the sidebar whose toolbar an anchored card
covers, top-left is the Servers header — so it is centered instead, which is
the one placement that reads as addressed to the window rather than attached to
the wrong panel.

Deliberately WITHOUT an overlay. This is a notification, not a decision that
must be made now, and blocking the page would force a choice between reading
the requests that explain the failure and keeping the affordance — dismissing
is not free, since "Authorize again" also clears the stale OAuth state, which a
plain reconnect does not do.

Kept as the existing declarative component rather than routed through Mantine's
`notifications`: that module is mocked in App.test.tsx, so switching would have
broken the ten tests asserting on this banner's text and its action button,
including the lost-authorization-state coverage. The message now stacks above a
right-aligned action, which a ~420px card needs.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/App.tsx

@@ -422,20 +422,41 @@ const ToastLinkButton = Anchor.withProps({
   size: "sm",
 });
 
-// Sticky re-auth banner bar. A `Paper` so every static style is a prop: `shadow`
-// emits `var(--mantine-shadow-sm)` (identical to the old CSS), and the stacking
+// The re-auth popup. A `Paper` so every static style is a prop; the stacking
 // order goes through `styles.root` since Mantine has no `z` prop.
+//
+// Floats top-right rather than spanning the top as a sticky full-bleed bar.
+// The bar cost the whole view a band of vertical space for what is a
+// notification about one server, and it sat directly above the monitoring
+// sidebar that an OAuth failure now opens (#2108) — the two things a user needs
+// at once here are this affordance and those requests, so it must not push them
+// around. `fixed`, not `sticky`, so scrolling the server list leaves it put.
+//
+// Centered, and deliberately WITHOUT an overlay. Every corner is spoken for —
+// the toast stack owns bottom-right (see `main.tsx`), the right edge is the
+// monitoring sidebar whose toolbar this would cover, and top-left is the
+// Servers header — so anchoring it anywhere hides something. Centering is the
+// one placement that reads as addressed to the whole window rather than
+// attached to the wrong panel.
+//
+// The missing overlay is the point, not an omission: this is a notification,
+// not a decision that must be made now. An OAuth failure opens the monitoring
+// sidebar (#2108), and blocking the page would force a choice between reading
+// those requests and keeping the affordance — dismissing is not free, since
+// "Authorize again" also clears the stale OAuth state, which a plain reconnect
+// does not do. So it floats above the page and leaves it usable.
+//
+// `transform` goes through `styles.root` for the same reason `zIndex` does:
+// Mantine exposes neither as a style prop.
 const ReAuthBannerBar = Paper.withProps({
-  px: "md",
-  pt: "xs",
-  pos: "sticky",
-  top: 60,
+  pos: "fixed",
+  top: "50%",
+  left: "50%",
+  w: 420,
   bg: "var(--mantine-color-body)",
-  shadow: "sm",
-  styles: { root: { zIndex: 200 } },
-  // Paper's default `radius: "md"` would round this full-bleed sticky bar's
-  // corners; the bar it replaced (a Box) had none.
-  radius: 0,
+  shadow: "xl",
+  radius: "md",
+  styles: { root: { transform: "translate(-50%, -50%)", zIndex: 200 } },
 });
 
 // Body of the "response body dropped" warning toast: a one-line summary of what