feat(dbviewer): full-text-search toggle wired to FTS5 query API

New UseFullTextSearch transient UI bool flips DbViewer.Filter() between
the existing local page filter (default) and the FTS5 MATCH path across
the whole database. ImRaii.Disabled blocks the toggle while the bulk-insert
worker is still building the index; the HelpMarker swaps between two
hints, one for the indexing state and one for the phrase-match advisory
once the index is ready.

Three new HellionStrings entries cover EN + DE + the Designer accessor:
- DbViewer_FullTextToggle (label)
- DbViewer_FullTextToggle_Hint_Indexing (tooltip while indexing)
- DbViewer_FullTextToggle_Hint_PhraseMode (tooltip once ready, warns
  multi-word terms match as phrases and how to opt into raw MATCH syntax)

Filter() short-circuits to the local fallback if the toggle is on but
ftsReady has flipped back to false -- defensive against a mid-session
Dispose-and-reopen during indexing.

v1.4.8 H2 Sub-Task 4.4.

HellionChat/Resources/HellionStrings.Designer.cs

@@ -402,4 +402,9 @@ internal class HellionStrings
 
     // Hellion Chat — v1.3.0 Honorific title slot tooltip
     internal static string ChatHeader_HonorificTitle_Tooltip => Get(nameof(ChatHeader_HonorificTitle_Tooltip));
+
+    // Hellion Chat — v1.4.8 DbViewer full-text search toggle
+    internal static string DbViewer_FullTextToggle => Get(nameof(DbViewer_FullTextToggle));
+    internal static string DbViewer_FullTextToggle_Hint_Indexing => Get(nameof(DbViewer_FullTextToggle_Hint_Indexing));
+    internal static string DbViewer_FullTextToggle_Hint_PhraseMode => Get(nameof(DbViewer_FullTextToggle_Hint_PhraseMode));
 }

HellionChat/Resources/HellionStrings.de.resx

@@ -917,4 +917,13 @@
   <data name="ChatHeader_HonorificTitle_Tooltip" xml:space="preserve">
     <value>Custom-Titel von Honorific</value>
   </data>
+  <data name="DbViewer_FullTextToggle" xml:space="preserve">
+    <value>Volltext-Suche</value>
+  </data>
+  <data name="DbViewer_FullTextToggle_Hint_Indexing" xml:space="preserve">
+    <value>Der Volltext-Index wird noch gebaut. Die lokale Suche bleibt verfügbar.</value>
+  </data>
+  <data name="DbViewer_FullTextToggle_Hint_PhraseMode" xml:space="preserve">
+    <value>Sucht nach der exakten Wortfolge. Mehrere Wörter werden nur gefunden, wenn sie zusammen und in dieser Reihenfolge stehen. Wer rohe FTS5-MATCH-Syntax nutzen will, setzt eigene Anführungszeichen um den Suchbegriff.</value>
+  </data>
 </root>

HellionChat/Resources/HellionStrings.resx

@@ -917,4 +917,13 @@
   <data name="ChatHeader_HonorificTitle_Tooltip" xml:space="preserve">
     <value>Custom title from Honorific</value>
   </data>
+  <data name="DbViewer_FullTextToggle" xml:space="preserve">
+    <value>Full-text search</value>
+  </data>
+  <data name="DbViewer_FullTextToggle_Hint_Indexing" xml:space="preserve">
+    <value>The full-text index is still being built. The local filter remains available.</value>
+  </data>
+  <data name="DbViewer_FullTextToggle_Hint_PhraseMode" xml:space="preserve">
+    <value>Searches for the exact phrase. Multi-word queries match only when the words appear together in order. To use raw FTS5 MATCH syntax, wrap your term in double quotes yourself.</value>
+  </data>
 </root>

HellionChat/Ui/DbViewer.cs

@@ -33,6 +33,12 @@ public class DbViewer : Window
 
     private int CurrentPage = 1;
     private string SimpleSearchTerm = "";
+
+    // v1.4.8 H2: opt-in full-text search across the whole DB via FTS5.
+    // Transient UI state (per-session), not persisted -- users opt in fresh
+    // every time so they always see the page-filter as the default mode.
+    private bool UseFullTextSearch;
+
     private bool OnlyCurrentCharacter = true;
     private readonly Dictionary<ChatType, (ChatSource, ChatSource)> SelectedChannels;
 
@@ -233,6 +239,24 @@ public class DbViewer : Window
             tooltipRight: Language.Page_ArrowRight_Tooltip
         );
 
+        // Full-text search toggle (v1.4.8 H2). IsFtsIndexBuilt is a cached
+        // volatile bool in MessageStore -- single field read per frame, no
+        // SELECT count(*). ImRaii.Disabled blocks any click while the index
+        // is still being built, so no defensive force-off branch needed
+        // inside the if-body. UseFullTextSearch is transient UI state, so we
+        // do not call SaveConfig here.
+        var ftsReady = Plugin.MessageManager.Store.IsFtsIndexBuilt;
+        using (ImRaii.Disabled(!ftsReady))
+        {
+            if (ImGui.Checkbox(HellionStrings.DbViewer_FullTextToggle, ref UseFullTextSearch))
+                Filtered = Filter(Messages);
+        }
+        ImGuiUtil.HelpMarker(
+            ftsReady
+                ? HellionStrings.DbViewer_FullTextToggle_Hint_PhraseMode
+                : HellionStrings.DbViewer_FullTextToggle_Hint_Indexing
+        );
+
         ImGui.SameLine(ImGui.GetContentRegionMax().X - width);
         ImGui.SetNextItemWidth(width);
         if (
@@ -452,6 +476,20 @@ public class DbViewer : Window
         if (SimpleSearchTerm == "")
             return new ConcurrentStack<Message>(messages.Reverse().OrderByDescending(m => m.Date));
 
+        // Full-text mode bypasses the page-bounded messages array and queries
+        // the FTS5 index across the whole DB. IsFtsIndexBuilt re-check guards
+        // against the (rare) case of the toggle being on while the index is
+        // mid-rebuild -- ImRaii.Disabled prevents the user from flipping it,
+        // but a Dispose-and-reopen during indexing could leave UseFullTextSearch
+        // true while ftsReady flipped back to false; the local fallback below
+        // still serves the page.
+        if (UseFullTextSearch && Plugin.MessageManager.Store.IsFtsIndexBuilt)
+        {
+            var hexIds = Plugin.MessageManager.Store.FullTextSearch(SimpleSearchTerm);
+            var resolved = Plugin.MessageManager.Store.LoadByGuids(hexIds);
+            return new ConcurrentStack<Message>(resolved.OrderByDescending(m => m.Date));
+        }
+
         return new ConcurrentStack<Message>(
             messages
                 .Reverse()