ci: auto-label issues by platform

[loss-and-quick]

Summary

Tags issues by platform automatically. GitHub Issue Forms can't set a label from a
dropdown answer, so a small workflow reads the Platform field of a bug report and
applies the matching platform:{android,linux,windows} label (keeping exactly one in
sync on open/edit). The bug form also gains a Windows desktop option and
Windows-specific OS/log hints.

The three platform:* labels are created in the repo (out of band, via the API).

Affected layer

• frontend/ — React Web UI
• module/ — Magisk/KernelSU/APatch payload (shell + packaged kasumi-proxy backend)
• scripts/ — build / release helpers
• CI / .github/
• Docs only

Verification

• actionlint clean on the new label-platform.yml.
• Issue form (bug_report.yml) parses as valid YAML; the Platform dropdown now lists
  Android / Linux / Windows.
• The workflow keys off the rendered ### Platform section of the issue body and is
  idempotent (removes a stale platform:* before adding the current one).

Checklist

• Title is a scoped Conventional Commit; commits are logically split
• No build artifacts committed
• If user-visible strings changed: n/a — issue-form copy only, no app i18n
• Renames touching the project id were grepped in all case forms — n/a

Notes for reviewers

• Labels render as the platform "badges" on each issue. Created with distinct colours
  (Android green, Linux amber, Windows blue).
• Independent of the Windows-support PR; the form lists Windows so reports can be filed
  against it as that lands.