add mqtt control

README.md

@@ -117,6 +117,7 @@ Supported keys and values in the JSON configuration are:
 * `opacity` : the same as the command line `-o` argument
 * `blur` : the same as the command line `-b` argument
 * `debug` : set to true to enable verbose output from the program
+* `mqtt` : MQTT playback control (see below)
 * `immich` : connect to an Immich server instead of a local path (see below)
 * `scheduler` : this entry is an array of possible path values and associated settings. This key lets you manage display times/settings for a collection of paths. In the example above the top entry shows ONLY files from a Redit feed between 2 and 4pm, ONLY files from the `show_peak_times` folder from 8am to 10am and then 4pm to 7pm. At all other times it alternates displaying files in the `always_show_1` and `always_show_2` folder.
    * `exclusive` : When set to `true` only this entry will be used when it is in its valid time window. 
@@ -124,9 +125,36 @@ Supported keys and values in the JSON configuration are:
    * `path` : the path to image files
    * `stretch` : as above
 
+### MQTT control
+
+Add an `mqtt` block to control playback remotely. Publish one of the commands below to the configured topic.
+
+Example:
+```
+{
+  "mqtt": {
+    "host": "mqtt.local",
+    "port": 1883,
+    "topic": "slide/control",
+    "clientId": "slide-frame",
+    "username": "slide",
+    "password": "secret",
+    "keepAlive": 30,
+    "qos": 0
+  }
+}
+```
+
+Commands:
+* `play` / `resume` — resume slideshow
+* `pause` — pause slideshow
+* `next` / `next-image` — advance to next image
+* `next-folder` — jump to next configured path (if multiple paths are configured)
+* `restart` / `reset` — recreate the selector and restart playback
+
 ### Immich configuration (lightweight + low power)
 
-Immich uses an API key and a `/api` base path. This integration requests the asset search endpoint and downloads thumbnail images into a local cache before displaying them. That keeps bandwidth and power usage low while still letting `slide` do its normal scaling and transitions.
+Immich uses an API key and a `/api` base path. This integration requests the asset search endpoint and downloads the configured image size into a local cache before displaying them. That keeps bandwidth and power usage low while still letting `slide` do its normal scaling and transitions.
 
 Example (single source):
 ```
@@ -171,7 +199,7 @@ Immich settings:
 * `order`: `"asc"` or `"desc"` ordering for asset search.
 * `pageSize`: assets fetched per page.
 * `maxAssets`: cap on total assets fetched (0 means no cap).
-* `cachePath`: local cache directory for downloaded thumbnails.
+* `cachePath`: local cache directory for downloaded images.
 * `cacheMaxMB`: maximum cache size in MB (0 disables cleanup).
 * `includeArchived`: include archived assets in search results.
 When `immich` is set on an entry, `path` and `imageList` are ignored.
@@ -205,6 +233,7 @@ See the `Configuration File` section for details of each setting.
 * qt5
 * qt5-image-formats-plugins
 * libexif
+* libmosquitto-dev
 
 Ubuntu/Raspbian: