feat: add required checkbox to confirm issue search before submission

Adds a pre-submission checklist to both bug report and feature request
templates requiring users to confirm they have searched for existing
issues before opening a new one.

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -17,6 +17,13 @@ body:
            take a moment to check that your issue hasn't been reported before.
         4. Please give all relevant information below for bug reports, because
            incomplete details will be handled as an invalid report and closed.
+  - type: checkboxes
+    id: searched
+    attributes:
+      label: Pre-submission checklist
+      options:
+        - label: I have searched for existing open or closed issue reports with the same problem.
+          required: true
   - type: textarea
     id: description
     attributes:

.github/ISSUE_TEMPLATE/feature.yml

@@ -15,6 +15,13 @@ body:
         2. Please ask questions or configuration/deploy problems on our [Matrix Room](https://matrix.to/#/#vikunja:matrix.org) or forum (https://community.vikunja.io).
         3. Make sure you are using the latest release and take a moment to check that your feature hasn't been requested before.
         4. Please include all relevant information in the feature request to allow users to discuss this fully.
+  - type: checkboxes
+    id: searched
+    attributes:
+      label: Pre-submission checklist
+      options:
+        - label: I have searched for existing open or closed issue reports with the same feature request.
+          required: true
   - type: textarea
     id: description
     attributes: