Add the UI toggle for changing theme

[dhruvkb]

Fixes

Fixes #4309 by @zackkrida
Fixes #4155 by @fcoveram

Description

This PR

• adds icon assets and i18n strings for theme switching
• updates the VSelectField component to hide the active choice
• creates the UI component for switching between color modes
  • wires the component to functionality implemented in Add color mode to ui store #4810
  • adds the UI component to the footer based on whether the "dark_mode_ui_toggle" feature flag is enabled
• deletes the "force_dark_mode" feature flag as it is redundant
  • deletes usage and tests for the feature flag

Testing Instructions

0. Check out the PR and run the dev server.
1. Enable the "dark_mode_ui_toggle" feature flag. You should see the theme selector in the footer.
2. Switch between light and dark theme. The site should update to show the your chosen color theme (regardless of the system theme). The icon in the footer should also update.
3. Switch between the OS light and dark theme. The text for the "System" choice in the theme selector should update.
4. Switch to the "System" theme.
5. Switch between the OS light and dark theme. The text for the "System" choice in the theme selector should update. The actual dark/light theme should also update. The icon in the footer should also update.

Checklist

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[dhruvkb]

This PR is drafted till #4810 is merged and labelled as "⛔️ status: blocked" (to prevent accidentally merging it). Once that is merged, it will be rebased and undrafted. Feel free to review in the meantime, I do not anticipate UI/functionality changes.

[zackkrida]

Lovely! There are a few small issues I'm noticing:

• The "system" mode icon doesn't reflect the user's preference on SSR, it is always light.
• There's an icon sizing/spacing issue:
  
• For the first issue, perhaps making the component clientonly would fix it

I really like how you're displaying the parenthetical of what the actual system value is, nice touch!

[dhruvkb]

@zackkrida that is so weird, I'm not seeing the select field sizing issue on any major browser! This is Safari, Arc (Chromium) and Firefox:

Safari has some wildly different problems going on, though.

[zackkrida]

@dhruvkb it looks like removing overflow-hidden from the VSelectField wrapper div fixes the collapsing issue. The problem makes sense to me: a flex child with overflow-hidden and two absolutely-positioned children would collapse, as I understand.

I'm seeing it in Chrome and Firefox on Fedora. In any case, I think we can safely remove the overflow hiding.

[obulat]

Nice to be able to change the theme from the UI!

I have 2 blocking requests:

1. The VThemeSelect should not detect the user's color scheme. It should only use the value from the UI store, and update it.
   app.vue (and error.vue) as the entrypoints to the application would be the best place for the detection of the user's color scheme. We update the UI store layout properties (breakpoint and width) there.

2. The select inside the VSelectField component does not set the outline style or width on focus-visible, but we do set its color. So with the default values, the focus ring has a white outline. Adding a focus-visible:outline-none to the select element could fix this white ring around the pink one:

[dhruvkb]

1. I need some more information here.

   The UI store only holds the user's choice of color theme, which can be dark/light/system. The first two correspond to actual themes but system can map to either of these. The component detects the user's theme to determine the icon to show. For the user's preference, it still reads from and writes to the UI store.

2. I cannot see the white outline.

   Firefox:
   

   Chrome:
   

[obulat]

The functionality is perfect ✨ It's nice that the composable encapsulates handling of the color mode and we don't have to update the pages.

In dark mode, I can see a ring around the select field focus border in Arc, Chrome and Safari. Firefox is the only browser that displays the focus border correctly. I'm not blocking on this because this is a problem that existed with the language select, and should have a separate issue.

Feedback implemented