4.1.4: QR sharing, venv tarball, flash script improvements

QR Channel Key Sharing:
- Admin-only QR generator in about.html (was visible to all)
- QR button for saved keys on account page
- Fixed about() route missing channel status vars (bug)

Pi Build Optimization:
- Pre-built venv tarball support (39MB zstd, skips 20+ min compile)
- setup.sh auto-detects and extracts tarball if present
- Strip __pycache__/tests before tarball (295MB → 208MB)

Flash Script Improvements:
- flash-image.sh now uses config.json for headless WiFi setup
- Consistent wipe prompt on both flash scripts
- pull-image.sh re-enables auto-expand before shrinking

Build Docs:
- Added zstd and jq to pre-setup apt-get
- Documented fast build option with pre-built venv

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

rpi/BUILD_IMAGE.md

@@ -26,8 +26,8 @@ ssh admin@stegasoo.local
 # Take ownership of /opt (for pyenv, jpegio builds)
 sudo chown admin:admin /opt
 
-# Install git (not included in Lite image)
-sudo apt-get update && sudo apt-get install -y git
+# Install git and zstd (not included in Lite image)
+sudo apt-get update && sudo apt-get install -y git zstd jq
 ```
 
 ## Step 4: Clone & Run Setup
@@ -39,7 +39,22 @@ cd stegasoo
 ./rpi/setup.sh
 ```
 
-This takes ~15-20 minutes and installs:
+### Fast Build Option (with pre-built venv)
+
+If you have `stegasoo-venv-pi-arm64.tar.zst` from a previous build:
+
+```bash
+cd /opt
+git clone -b 4.1 https://github.com/adlee-was-taken/stegasoo.git stegasoo
+
+# Copy pre-built venv (from your host machine)
+# On host: scp rpi/stegasoo-venv-pi-arm64.tar.zst admin@stegasoo.local:/opt/stegasoo/rpi/
+
+cd stegasoo
+./rpi/setup.sh  # Detects tarball, extracts instead of compiling (~2 min vs 20+)
+```
+
+**Standard build** takes ~15-20 minutes and installs:
 - Python 3.12 via pyenv
 - jpegio (patched for ARM)
 - Stegasoo with web UI