refactor(ux): address /code-review high findings on the fixpack

- Extract the optimistic + debounced-flush feed-priority flow into a shared
  useChannelPriority hook (mirrors useChannelTagActions); the manager and the
  channel-detail page now share it instead of two drifting copies.
- NavSidebar: drop the always-true `slim` const and its dead expanded-rail
  branches; refresh the now-stale pinned/collapsible comments.
- Remove the orphaned `navCollapsed` localStorage key (no readers/writers left
  after the always-slim rail).
- Channel page: drop the wrapper native `title` that duplicated PriorityStepper's
  own Tooltip.

The keyboard-focus nav-label finding needed no change: the popover platform
already wires useFocus (visibleOnly), so a real Tab press shows the flyout label
(verified in-browser).

frontend/src/components/NavSidebar.tsx

@@ -17,9 +17,9 @@ import Tooltip from "./Tooltip";
 import LanguageSwitcher from "./LanguageSwitcher";
 import { PopoverSurface, useAnchoredPopover } from "./AnchoredPopover";
 
-// Primary app navigation: a collapsible left rail with icon+label entries (Design C). The
-// modules used to hide under the avatar dropdown; they now live here. Collapsed, it becomes
-// a thin icon-only rail. Account actions sit at the bottom in a popover.
+// Primary app navigation: a thin icon-only left rail. The modules used to hide under the avatar
+// dropdown; they now live here, each revealing its name in a hover/focus flyout label. Account
+// actions sit at the bottom in a popover.
 export default function NavSidebar({
   me,
   onOpenAbout,
@@ -34,10 +34,10 @@ export default function NavSidebar({
   const { t } = useTranslation();
   const { page } = useNavigation();
   const { setPage } = useNavigationActions();
-  // The rail is ALWAYS slim: a thin icon-only column flush against the left edge. There is no
-  // expanded/pinned mode anymore — instead each icon reveals a floating name label on hover (a
-  // rest-to-show flyout via Tooltip), so the names are one glance away without the rail ever growing.
-  const slim = true;
+  // The rail is ALWAYS a thin icon-only column flush against the left edge. There is no
+  // expanded/pinned mode anymore — instead each icon reveals a floating name label on hover or
+  // keyboard focus (a rest-to-show flyout via Tooltip), so the names are one glance away without the
+  // rail ever growing.
   const [acctOpen, setAcctOpen] = useState(false);
   // The account panel holds mixed content with its own tab order (profile, account switches, two
   // actions), so it is a `dialog` popover, not a roving menu. It portals out of the nav — whose
@@ -198,8 +198,6 @@ export default function NavSidebar({
   };
 
   return (
-    // Layout slot: reserves only the slim footprint when unpinned, so the expanded rail (below)
-    // overlays content instead of pushing it. Pinned reserves the full width (in-flow).
     // Slim icon rail, flush against the left viewport edge (only top/bottom margin), never grows.
     // The slot is exactly the card's width so the filter tab/bar docks flush against its right edge.
     <div className="relative shrink-0 w-14">
@@ -213,28 +211,15 @@ export default function NavSidebar({
           style={listFade.style}
         >
           {userItems.map(renderItem)}
-          {systemItems.length > 0 &&
-            (slim ? (
-              // Collapsed rail: a short, thicker centred rule is the clearest "new section" cue
-              // when there's no room for a label.
-              <div className="my-2 mx-auto w-7 border-t-2 border-border" />
-            ) : (
-              <div className="mt-4 mb-1 px-2.5 flex items-center gap-2">
-                <span className="text-3xs font-semibold uppercase tracking-wider text-muted/70">
-                  {t("nav.adminSection")}
-                </span>
-                <span className="flex-1 border-t border-border" />
-              </div>
-            ))}
+          {systemItems.length > 0 && (
+            // A short, thicker centred rule is the clearest "new section" cue on an icon-only rail.
+            <div className="my-2 mx-auto w-7 border-t-2 border-border" />
+          )}
           {systemItems.map(renderItem)}
         </div>
 
         <div className="mt-2 pt-2 border-t border-border flex flex-col gap-1">
-          <div
-            className={`flex items-center justify-center gap-2 pb-2 mb-1 border-b border-border ${
-              slim ? "flex-col" : "flex-row"
-            }`}
-          >
+          <div className="flex flex-col items-center justify-center gap-2 pb-2 mb-1 border-b border-border">
             <LanguageSwitcher variant="rail" value={language} onChange={onChangeLanguage} />
             <button
               onClick={onOpenAbout}