feat(l2): screenshots over USB (RGB565); USB tools moved to the Solo site

- CMD_GET_SCREENSHOT on colour screens: display type 2 (RGB565 LE),
  32-bit payload size; ui-lvgl captureFrame() shared with the SD BMP
- ENABLE_SCREENSHOT in the L2 build; tools/screenshot.py decodes type 2
- docs and READMEs point to solo.marekzegarek.com/#pc-tools
- USB popup: "Keep card" instead of "Charge only" (only the card is lent;
  charging and USB serial work either way)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/solo/hardware.md

@@ -84,8 +84,9 @@ spare the panel.
 - **SD card**: holds the message history, maps, GPX trails and live map tiles.
   Settings › Storage shows what takes the space, how many messages each
   conversation keeps, and deletes the history.
-- **USB drive**: plugged into a computer, the device asks whether to only
-  charge or to lend the computer the SD card. While lent, the device can't use
+- **USB drive**: plugged into a computer, the device asks whether to keep the
+  SD card or lend it to the computer. Charging and the USB connection (the
+  site's USB tools, the app) work either way. While lent, the device can't use
   the card; eject it on the computer and the device restarts. With a screen
   PIN set, it doesn't ask until the screen is unlocked.
 - **WiFi**: used only for map downloads, live tiles and updates, and off the

docs/solo/navigation.md

@@ -29,7 +29,7 @@ to keep it through a reboot.
 
 ### GPX export
 
-Connect USB, open [Solo Tools](https://marekzegare4.github.io/Solo-tools/) in
+Connect USB, open the [USB tools](https://solo.marekzegarek.com/#pc-tools) on the Solo site in
 Chrome or Edge and click **Connect device**, then on the device choose
 **Trail file › Export**. The GPX includes your waypoints. Without a browser,
 `uv run tools/trail_export.py` does the same. Disconnect the phone app first if