fix: switch issue templates to YAML forms, remove broken markdown templates

Markdown-based issue templates with frontmatter were silently ignored
by GitHub (likely an empty-string assignees field or frontmatter
format issue). Switched to the newer YAML-based issue forms which use
a structured schema format that GitHub explicitly supports.

- bug_report.yml: structured form with required fields for description,
  reproduction steps, environment, screenshots
- feature_request.yml: form with problem/motivation, proposed solution,
  alternatives, mockups
- config.yml: unchanged, keeps blank_issues_enabled: false

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,43 @@
+name: ✨ Feature request
+description: Suggest an idea, enhancement, UX improvement, or refactor
+title: '[enhancement] '
+labels: ['enhancement']
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for your suggestion! Please describe the feature or improvement you'd like to see.
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem / motivation
+      description: What problem would this solve? Why is this needed?
+      placeholder: |
+        e.g. It's frustrating when ... or The app currently lacks ...
+    validations:
+      required: true
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed solution
+      description: A clear description of what you want to happen.
+      placeholder: Describe your ideal solution here.
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternative approaches
+      description: Any alternative solutions or features you've considered.
+      placeholder: What else could work?
+  - type: textarea
+    id: mockups
+    attributes:
+      label: Screenshots / mockups
+      description: Sketches, screenshots from other apps, or wireframes.
+      placeholder: Drag & drop images here
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional context
+      description: Any other details, links, or relevant API endpoints.