feat: add ext: operator for file extension filtering in advanced search

- Add ext: operator support to query parser in backend and frontend
- Update search documentation in README and help modal with ext: examples
- Parse ext: operator to extract file extension filter (strips leading dot, converts to lowercase)
- Filter search candidates by file extension in advanced_search function
- Add ext chip display in search UI alongside existing tag/vault/title/path chips
- Update API documentation and function

frontend/app.js

@@ -305,11 +305,11 @@
   };
 
   // ---------------------------------------------------------------------------
-  // Query Parser — extracts operators (tag:, #, vault:, title:, path:)
+  // Query Parser — extracts operators (tag:, #, vault:, title:, path:, ext:)
   // ---------------------------------------------------------------------------
   const QueryParser = {
     parse(raw) {
-      const result = { tags: [], vault: null, title: null, path: null, freeText: "" };
+      const result = { tags: [], vault: null, title: null, path: null, ext: null, freeText: "" };
       if (!raw) return result;
       const tokens = this._tokenize(raw);
       const freeTokens = [];
@@ -326,6 +326,8 @@
           result.title = tok.slice(6).replace(/"/g, "").trim();
         } else if (lower.startsWith("path:")) {
           result.path = tok.slice(5).replace(/"/g, "").trim();
+        } else if (lower.startsWith("ext:")) {
+          result.ext = tok.slice(4).replace(/"/g, "").trim().replace(/^\./, "").toLowerCase();
         } else {
           freeTokens.push(tok);
         }
@@ -657,6 +659,10 @@
         this._addChip("path", `path:${parsed.path}`, parsed.path);
         hasChips = true;
       }
+      if (parsed.ext) {
+        this._addChip("ext", `ext:${parsed.ext}`, parsed.ext);
+        hasChips = true;
+      }
       this._container.hidden = !hasChips;
     },
     clear() {