feat(ui): warn user if FFmpeg not installed (#441)

* feat: Warn user if FFmpeg is not installed

Creates a Warning dialog on startup if the program cannot find FFmpeg or
FFprobe in the PATH. Other interactions with the program are blocked
until the issue is either ignore or resolved.

* docs: Add FFmpeg installation guide

* ruff formatting

* chore: Cleanup missing logic and warning message

* chore: Remove custom icon

Per QT docs, handling custom iconPixmap requires multiple icons per
platform. Easier to just use universal, default warning icon (yellow
triangle)

* fix: Ignore dialog with X button

* fix: Move startup checks after CI

* chore: Unreverse install check logic

* doc: Improve docs formatting

* docs: Point help url to new docs sites

* Remove ffmpeg docs page

* Use which from python stdlib

.github/workflows/apprun.yaml

@@ -33,7 +33,8 @@ jobs:
             libxcb-xinerama0 \
             libopengl0 \
             libxcb-cursor0 \
-            libpulse0
+            libpulse0 \
+            ffmpeg
 
       - name: Install dependencies
         run: |