feat(tabs): add IsPinned with separate pool and 5-tab cap

Tester-Request from Jin (2026-05-03): TempTabs should be pinnable so a
key conversation partner survives a relog. Right-click a TempTab and
choose Pin Tab / Unpin Tab / Promote to permanent.

Pool semantics:
- AutoTellTabsLimit (15) still gates the auto-managed unpinned pool.
- Pinned TempTabs live in their own pool, hard-capped at 5.
- The 6th pin attempt fails with a notification; users can unpin first
  or promote to permanent.
- Unpinning into a full unpinned pool drops the oldest unpinned (no
  user friction).

Mechanics:
- Tab.IsPinned (default false); Tab.Clone() carries it.
- Migration v16 -> v17 (additive; existing tabs default to unpinned).
- Three strip-sites synchronised through TabLifecycleHelpers:
  Plugin.cs load-time, Plugin.SaveConfig, Configuration.UpdateFrom.
- AutoTellTabsService:
  * MaxPinnedTempTabs constant.
  * F2.1 _activeTempTabCount counter retired — ActiveTempTabCount is
    now Tabs.Count(predicate). Pin/Unpin/Promote transitions are
    cold-path and don't need lock-free reads.
  * DropOldestTempTab filters on IsInUnpinnedPool so pinned tabs are
    never drop candidates.
  * OnLogout strips only the unpinned pool; pinned popouts and the
    active-tab switch behave correspondingly.
  * TryPin / Unpin / PromoteToPermanent service methods.
- ChatLogWindow tab context menu: Pin / Unpin / Promote with disabled-
  state at-cap tooltip + Promote tooltip explaining the channel-filter
  side effect.
- HellionStrings (EN+DE) for menu labels, tooltips, the limit warning.
- AutoTellTabsLimit slider description now flags the separate pinned
  pool so users aren't surprised by 18 tabs when the limit reads 15.

HellionChat/Ui/ChatLogWindow.cs

@@ -2124,10 +2124,56 @@ public sealed class ChatLogWindow : Window
             anyChanged = true;
         }
 
+        if (tab.IsTempTab)
+        {
+            ImGui.Separator();
+            DrawPinControls(tab);
+        }
+
         if (anyChanged)
             Plugin.SaveConfig();
     }
 
+    private void DrawPinControls(Tab tab)
+    {
+        var svc = Plugin.AutoTellTabsService;
+        if (svc == null)
+            return;
+
+        if (tab.IsPinned)
+        {
+            if (ImGui.MenuItem(HellionStrings.PinTab_MenuUnpin))
+            {
+                svc.Unpin(tab);
+                ImGui.CloseCurrentPopup();
+            }
+        }
+        else
+        {
+            var atCap = svc.PinnedTempTabCount >= AutoTellTabsService.MaxPinnedTempTabs;
+            if (ImGui.MenuItem(HellionStrings.PinTab_MenuPin, enabled: !atCap))
+            {
+                if (svc.TryPin(tab))
+                    ImGui.CloseCurrentPopup();
+            }
+            if (atCap && ImGui.IsItemHovered(ImGuiHoveredFlags.AllowWhenDisabled))
+                ImGui.SetTooltip(
+                    string.Format(
+                        HellionStrings.PinTab_LimitReached,
+                        AutoTellTabsService.MaxPinnedTempTabs
+                    )
+                );
+        }
+
+        if (ImGui.MenuItem(HellionStrings.PinTab_MenuPromote))
+        {
+            svc.PromoteToPermanent(tab);
+            ImGui.CloseCurrentPopup();
+        }
+        if (ImGui.IsItemHovered())
+            ImGui.SetTooltip(HellionStrings.PinTab_PromoteTooltip);
+    }
+
     internal readonly List<bool> PopOutDocked = [];
     internal readonly HashSet<Guid> PopOutWindows = [];