Add daemon startup smoke test and let it run on a PC in debug_mode

v0.11.0 shipped a daemon binary that built fine but didn't actually serve
a working frontend. CI was green. Nothing in the pipeline asserted that
the built binary comes up and serves something.

Add daemon/tests/smoke.rs as an integration test that spawns the built
binary against a tempdir-backed config with debug_mode = true, picks an
ephemeral port, and asserts:

- GET /index.html is 2xx and the decompressed body contains "Rayhunter"
- GET /api/qmdl-manifest is 2xx
- the daemon exits cleanly on SIGINT

Captures the daemon's stderr into a buffer so startup/shutdown failures
print actionable context instead of just "did not start listening".

Runs as part of the regular cargo test invocation, no new CI job.

For the smoke test (and #826) to work, the daemon needs to come up on a
PC without /dev/diag, a screen, or wpa_supplicant. The DIAG read thread,
display driver, and key input were already gated on debug_mode. Gate the
two remaining device-dependent workers the same way:

- run_battery_notification_worker (polls battery sysfs paths)
- wifi_station::run_wifi_client (talks to wpa_supplicant)

doc/installing-from-source.md gains a "Running the daemon on your PC"
section. doc/porting.md drops its duplicate debug_mode line and links to
the new section.

Closes #826.

doc/installing-from-source.md

@@ -27,6 +27,25 @@ Then you can build everything with:
 ./scripts/install-dev.sh orbic  # replace 'orbic' with your device type
 ```
 
+## Running the daemon on your PC
+
+If you don't have a target device handy, you can run `rayhunter-daemon` on your
+PC with `debug_mode = true`. This skips DIAG, the device display, key input,
+the battery worker, and the WiFi client, so recording-related endpoints will
+not work, but the frontend and read-only APIs do.
+
+```sh
+mkdir -p ./qmdl && printf 'entries = []\n' > ./qmdl/manifest.toml
+cat > config.toml <<'EOF'
+qmdl_store_path = "./qmdl"
+port = 8080
+debug_mode = true
+EOF
+cargo run -p rayhunter-daemon -- ./config.toml
+```
+
+Open `http://127.0.0.1:8080`.
+
 ## Hot-reloading the frontend
 
 If you are working on the frontend, you normally have to repeat all of the above steps everytime to see a change.