refactor: remove lite variant, fix release workflow

- Remove all lite/full variant logic from frontend, API, shared constants,
  docs, and tests (single unified Docker image only)
- Replace single QEMU multi-arch Docker build with per-architecture native
  builds (amd64 + arm64) and manifest merge to fix disk space exhaustion
- Add disk cleanup step and per-platform build cache scopes
- Switch release trigger from push to workflow_dispatch
- Add GitHub issue templates and PR template

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,45 @@
+name: Feature Request
+description: Suggest a new tool, improvement, or change
+labels: ["enhancement"]
+body:
+  - type: checkboxes
+    id: checklist
+    attributes:
+      label: Before submitting
+      options:
+        - label: I searched existing issues and didn't find a duplicate
+          required: true
+
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem
+      description: What problem does this solve? Describe the use case or frustration.
+      placeholder: |
+        I frequently need to convert AVIF files to JPEG, but there's no tool for it.
+        I currently have to use an external service which means my images leave my machine.
+    validations:
+      required: true
+
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed solution
+      description: How do you think this should work?
+      placeholder: |
+        Add an AVIF option to the format conversion tool, similar to how
+        HEIC and WebP are already supported.
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: Have you tried any workarounds? Are there other approaches that could work?
+
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional context
+      description: Screenshots, links, examples, or anything else that helps explain the request.