feat(web,a11y): slider accessibility improvements (#8479)

* feat(web,a11y): slider accessibility improvements

* add perceivable focus outline
* label all sliders for screen readers

* chore: add IDs to all settings sliders

* chore: add comment to id prop

* fix: switch to using CSS to add outlines

* fix: reactive sliderId

* fix: bring back the slot

* fix: add aria-describedby for the subtitle

* fix: cleanup css because disabled slider cannot be focused

* fix: add border to the slider when focus is visible

---------

Co-authored-by: Alex Tran <alex.tran1502@gmail.com>

web/src/lib/components/admin-page/settings/ffmpeg/ffmpeg-settings.svelte

@@ -232,6 +232,7 @@
         />
 
         <SettingSwitch
+          id="two-pass-encoding"
           title="TWO-PASS ENCODING"
           {disabled}
           subtitle="Transcode in two passes to produce better encoded videos. When max bitrate is enabled (required for it to work with H.264 and HEVC), this mode uses a bitrate range based on the max bitrate and ignores CRF. For VP9, CRF can be used if max bitrate is disabled."
@@ -286,6 +287,7 @@
             />
 
             <SettingSwitch
+              id="temporal-aq"
               title="TEMPORAL AQ"
               {disabled}
               subtitle="Applies only to NVENC. Increases quality of high-detail, low-motion scenes. May not be compatible with older devices."