chore: point dev.sh at .venv, document Linux first-run setup

dev.sh now uses .venv/bin/python3 so it picks up the project's
virtualenv rather than the bare system Python (which has no deps).

CLAUDE.md gains a one-time setup block for fresh Linux/WSL machines:
create the venv, install backend deps, and reinstall frontend
node_modules (Windows-built native binaries don't work on Linux).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: AlpineArchitecture

CLAUDE.md

@@ -69,6 +69,15 @@ data/             Bind-mounted volume — config.json, channels.json, plex_libra
 Two loops: the **fast loop** for iterating, the **parity loop** (Docker) for the final
 check before shipping. Always run the parity loop before a release.
 
+**One-time setup (Linux/WSL — fresh machine):**
+```bash
+# Python venv (requires python3-venv: sudo apt install python3.14-venv)
+python3 -m venv .venv && .venv/bin/pip install -r backend/requirements.txt
+# Frontend — must reinstall on Linux even if node_modules exists from Windows
+# (Windows-built native binaries don't work cross-platform)
+cd frontend && npm install && cd ..
+```
+
 **Fast loop — hot reload:**
 ```bash
 # Linux/WSL:
@@ -78,7 +87,6 @@ check before shipping. Always run the parity loop before a release.
 ```
 Open **http://localhost:5173** (not 7979). Vite serves the SPA with HMR and proxies `/api`
 → the reload backend. Both read/write the real `./data` files, so behavior matches Docker.
-Requires a local Python env with backend `requirements.txt` and `npm install` in `frontend/`.
 
 **Parity loop — Docker (run before shipping):**
 ```powershell

dev.sh

@@ -24,7 +24,7 @@ echo "  Backend (reload): http://localhost:7979"
 echo "  Ctrl+C to stop both."
 echo ""
 
-python3 -m uvicorn main:app --reload --port 7979 --app-dir "$ROOT/backend" &
+"$ROOT/.venv/bin/python3" -m uvicorn main:app --reload --port 7979 --app-dir "$ROOT/backend" &
 cd "$ROOT/frontend" && npm run dev &
 
 wait