chore: add GitHub issue forms (bug + feature + config) (#517)

Three files under .github/ISSUE_TEMPLATE/:

- bug_report.yml      Required: summary, repro steps, expected/actual,
                      version, OS. Optional: install source dropdown
                      (AUR / .deb / .rpm / Cachix / .dmg / .msi /
                      from-source), Subsonic server type + version,
                      logs (with inline how-to), screenshots, anything
                      else. Auto-labels: bug, triage.

- feature_request.yml Required: use case (described as workflow, not
                      solution). Optional: proposed solution,
                      alternatives, anything else. Auto-labels:
                      enhancement, triage.

- config.yml          blank_issues_enabled: false; redirects general
                      questions to Discord, Telegram for chat,
                      AUR page for packaging issues.

Net effect: incoming issues land with the info needed to triage
without back-and-forth, and casual support questions are routed off
the issue tracker.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,43 @@
+name: Feature request
+description: Suggest a new feature or improvement
+title: "[Feature]: "
+labels: ["enhancement", "triage"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for suggesting an improvement. A short description of the use case is more
+        valuable than a fully-specified solution — the implementation usually shifts during
+        review, but the underlying need stays the same.
+
+  - type: textarea
+    id: problem
+    attributes:
+      label: What's the use case?
+      description: |
+        What are you trying to do that's currently hard or impossible? Describe the workflow
+        or situation, not the proposed solution.
+      placeholder: |
+        e.g. When I'm on a long flight I want to keep listening to my offline-cached tracks,
+        but if I open Psysonic without an internet connection it shows the connection
+        screen instead of letting me into the player.
+    validations:
+      required: true
+
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed solution
+      description: How do you imagine this working? Sketches, UI ideas, command names — anything goes.
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: Workarounds you've tried, or other approaches that didn't fit.
+
+  - type: textarea
+    id: anything_else
+    attributes:
+      label: Anything else
+      description: Links to related issues, similar requests, screenshots, etc.