perf(ci): use pre-built container image for Android builds

- Add .devcontainer/ci.Dockerfile: bakes Flutter, JDK 17, Android SDK,
  and flutter precache --android into a single Docker image (~2GB, built once)
- ci.yml: build-android uses kryz-ci:latest container — removes 8 redundant
  SDK download steps (Java 17, Flutter, Android SDK, precache, licenses)
  that account for ~35-40 min of the 40-min container spin-up time
- release.yml: build-android uses kryz-ci:latest container — same reduction
  from 13 steps to 8. Also fixes hardcoded 'token ***' -> '$GITEA_TOKEN'
- devcontainer.json: simplified postCreateCommand (removed venv, pip, claude)
  and cleaned up invalid JSON (comments, empty mounts array)

Estimated impact: 40min -> 5-10min first run (with pre-built image),
~2-3min subsequent runs (Docker layer caching)

.gitea/workflows/ci.yml

@@ -7,12 +7,9 @@
 # Lint/test use continue-on-error: true so Magrathea failures don't block.
 # Android build detects wrong platform and exits 0 gracefully.
 #
-# CONTAINER IMAGE: The build-android job uses a pre-built Docker image
-# (kryz-ci:latest) that includes Flutter, JDK 17, and Android SDK baked in —
-# eliminating 3-5 GB of downloads per job. Build once on the runner host:
-#   docker build -f .devcontainer/ci.Dockerfile -t kryz-ci:latest .devcontainer
-#
-# Estimated impact: 40min -> 5-10min (with pre-built image)
+# IMPORTANT: $GITHUB_PATH does NOT persist between steps in Gitea Actions/act.
+# Each step is a separate shell. We export PATH="$PWD/flutter/bin:$PATH" in
+# every step that uses flutter/dart commands.
 
 name: CI
 
@@ -118,6 +115,10 @@ jobs:
 
 
   # ── Stage 2: Android build (uses pre-built container image) ──────
+  # The kryz-ci:latest image contains Flutter, JDK 17, Android SDK, and
+  # flutter precache --android baked in — eliminating ~3-5 GB of downloads.
+  # Build once on the runner host:
+  #   docker build -f .devcontainer/ci.Dockerfile -t kryz-ci:latest .devcontainer
   build-android:
     name: Build Android
     runs-on: [self-hosted, linux, x64]