fix(layout): topbar grid auto-expanded center column hid right buttons at 780-1280

User reported the search bar dropping to a second row + the top-right
buttons (+ New / Inbox / Avatar) going missing as they resized the
browser. Playwright probe confirmed: at every width 780-1280 the
search bar's intrinsic `max-w-2xl` (672px) forced the topbar's
center grid column to expand to that width, leaving the right
column too narrow to hold "+ New + Inbox + Avatar" without
overlapping the search OR going off-screen.

Two coordinated fixes:

1. Grid template `auto_1fr_auto` instead of `1fr_minmax(280,800)_1fr`.
   Side columns now size to their actual content (logo + breadcrumbs
   on the left; New + Inbox + Avatar on the right); the center
   column takes whatever's left. No more "intrinsic content forces
   the column to grow" behaviour.

2. Search wrapper max-width scales by tier: max-w-md (448px) at
   base, lg:max-w-xl (576px), xl:max-w-2xl (672px). Generous enough
   on wide screens, restrained enough on narrow ones so the side
   columns always get the space they need.

Verified via Playwright probe at 780/900/1023/1024/1100/1280 —
"+ New" button now lands inside the header at every width.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/layout/topbar.tsx

@@ -60,12 +60,16 @@ export function Topbar({ ports, user, leadingSlot }: TopbarProps) {
     // The brand logo lives in the sidebar header (per design feedback) so the
     // topbar center is dedicated to the global search bar.
     //
-    // Center column min-width: 280 at tablet, 420 at lg+. The earlier
-    // single 420 min starved the left column to ~100px at 768 viewport
-    // width (when the sidebar is hidden under a Sheet on tablet) — that
-    // was hiding the new logo-trigger leadingSlot AppShell mounts. Two
-    // breakpoints split the difference cleanly.
-    <header className="grid h-14 grid-cols-[minmax(0,1fr)_minmax(280px,800px)_minmax(0,1fr)] items-center border-b border-border bg-background gap-3 px-4 shrink-0 lg:grid-cols-[minmax(0,1fr)_minmax(420px,800px)_minmax(0,1fr)]">
+    // Grid is `auto auto 1fr` instead of three fr-tracks: the left + right
+    // columns size to their actual content (logo trigger + breadcrumbs on
+    // the left; New / Inbox / Avatar on the right), and the search column
+    // soaks up the rest. The earlier `minmax(280px,800px)` center column
+    // auto-grew to the search bar's intrinsic `max-w-2xl` (672px), which
+    // squeezed the right column below the width of "+ New + Inbox +
+    // Avatar" and pushed the New button off-screen at every tablet +
+    // narrow-desktop width. With the center as a single fr-track, the
+    // right column always gets the space it needs.
+    <header className="grid h-14 grid-cols-[auto_1fr_auto] items-center border-b border-border bg-background gap-3 px-4 shrink-0">
       {/* LEFT: optional sidebar trigger (tablet) + optional back button + breadcrumbs */}
       <div className="min-w-0 flex items-center gap-1.5">
         {leadingSlot}
@@ -86,18 +90,18 @@ export function Topbar({ ports, user, leadingSlot }: TopbarProps) {
         <Breadcrumbs />
       </div>
 
-      {/* CENTER: global search - capped width and visually centered
-          against the FULL viewport (not the post-sidebar area) via a
-          translate-X that shifts left by half the sidebar width.
-          Without the translate the topbar's grid centers inside the
-          area-after-the-sidebar, so the search visually drifts right
-          by half the sidebar width.
-          The translate is gated to `lg:` because at tablet (768-1023)
-          the sidebar is HIDDEN behind a Sheet — translating left there
-          shifts the search into the leading-slot column, hiding the
-          AppShell-mounted logo trigger. */}
+      {/* CENTER: global search. Caps scale by viewport tier so the search
+          bar doesn't crowd the side columns at narrow widths:
+            base: max-w-md  (28rem / 448px) — fits tablet 768-1023 with
+                  ~150-200px left for the right column.
+            lg:   max-w-xl  (36rem / 576px) — narrow desktop with sidebar.
+            xl:   max-w-2xl (42rem / 672px) — full desktop, plenty of room.
+          The lg: translate-X visually centers the search against the FULL
+          viewport (compensating for the sidebar's 256px on the left). It
+          stays gated to lg+ so tablet (sidebar hidden behind Sheet)
+          doesn't get an unnecessary shift. */}
       <div className="flex items-center justify-center min-w-0">
-        <div className="w-full max-w-2xl mx-auto min-w-0 lg:-translate-x-[calc(var(--width-sidebar)/2)]">
+        <div className="w-full max-w-md mx-auto min-w-0 lg:max-w-xl xl:max-w-2xl lg:-translate-x-[calc(var(--width-sidebar)/2)]">
           <CommandSearch />
         </div>
       </div>