fix(search): reject empty phrases and phrases glued to other text

The lexer now keeps token offsets and rejects what the grammar never allowed
but the token rules accepted:
- an empty phrase: "" (matched every card), tag:""
- a phrase touching a word or another phrase: a"b", "a"b, "a""b"
- whitespace between a qualifier and its phrase value: tag: "x"

key:"value" stays the only legal contact; parentheses still delimit phrases.
Listed in PLAN 6.1/6.2 and USAGE.

YKanBan.Tests/Search/SearchQueryParserTests.cs

@@ -296,6 +296,42 @@ public class SearchQueryParserTests
         AssertSyntaxError("tag: bug");
     }
 
+    [TestMethod]
+    [DataRow("tag: \"x\"")]
+    [DataRow("title:  \"a b\"")]
+    public void SpacedQualifierPhraseValueIsAnError(string text)
+    {
+        AssertSyntaxError(text);
+    }
+
+    [TestMethod]
+    [DataRow("a\"b\"")]
+    [DataRow("\"a\"b")]
+    [DataRow("\"a\"\"b\"")]
+    [DataRow("tag:\"x\"y")]
+    [DataRow("tag:\"x\"\"y\"")]
+    public void PhraseGluedToOtherTextIsAnError(string text)
+    {
+        AssertSyntaxError(text);
+    }
+
+    [TestMethod]
+    [DataRow("\"\"")]
+    [DataRow("a \"\"")]
+    [DataRow("tag:\"\"")]
+    public void EmptyPhraseIsAnError(string text)
+    {
+        AssertSyntaxError(text);
+    }
+
+    [TestMethod]
+    public void PhraseMayTouchParentheses()
+    {
+        AssertParsesTo(
+            new SearchNode.And([new SearchNode.FreeText("a b"), new SearchNode.FreeText("c")]),
+            "(\"a b\")(c)");
+    }
+
     [TestMethod]
     [DataRow("id:12x")]
     [DataRow("id:abc")]