Remove ellipsis mixin and delete mixins.scss (#2480)

The `ellipsis()` mixin in `mixins.scss` was only used in two files.
Replaced with plain CSS and removed the file entirely.

## Changes

- **Converted mixin usage to plain CSS** in `flyoutmenu.scss` and
`expandablemenu.scss`:
  ```scss
  // Before
  .label {
      @include mixins.ellipsis();
  }
  
  // After
  .label {
      display: block;
      white-space: nowrap;
      overflow: hidden;
      text-overflow: ellipsis;
  }
  ```

- **Removed unused imports** from 8 SCSS files that imported
`mixins.scss` but didn't use it

- **Deleted** `frontend/app/mixins.scss`

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<details>

<summary>Original prompt</summary>

> in mixins.scss we have one last one called ellipsis(). i believe it is
only used in a couple places now... at least flyoutmenu.scss and
expandablemenu.scss. lets convert those to just plain CSS instead of
using the mixin. and then i think we can delete the mixins.scss file
completely... maybe 8 or so scss files that include it that will also
need modification.
> 
> do your own check though to find any additional usages of mixins.scss
and the ellipsis mixin. make sure we remove all usages.


</details>



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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: sawka <[email protected]>

Author: Copilot

frontend/app/block/block.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .block {
     display: flex;
     flex-direction: column;

frontend/app/element/button.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .wave-button {
     // override default button appearance
     border: 1px solid transparent;

frontend/app/element/expandablemenu.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .expandable-menu {
     display: flex;
     flex-direction: column;
@@ -20,7 +18,10 @@
     border-radius: 4px;
 
     .label {
-        @include mixins.ellipsis();
+        display: block;
+        white-space: nowrap;
+        overflow: hidden;
+        text-overflow: ellipsis;
     }
 }
 

frontend/app/element/flyoutmenu.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .menu {
     position: absolute;
     z-index: 1000;
@@ -36,7 +34,10 @@
     border-radius: 2px;
 
     .label {
-        @include mixins.ellipsis();
+        display: block;
+        white-space: nowrap;
+        overflow: hidden;
+        text-overflow: ellipsis;
         text-decoration: none;
     }
 }

frontend/app/element/input.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .input {
     width: 100%;
     border: none;

frontend/app/mixins.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../mixins.scss";
-
 .type-ahead-modal-backdrop {
     position: absolute;
     top: 0;

frontend/app/modals/typeaheadmodal.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../../mixins.scss";
-
 .csv-view {
     opacity: 0; /* Start with an opacity of 0, meaning it's invisible */
 

frontend/app/view/preview/csvview.scss

@@ -1,8 +1,6 @@
 // Copyright 2024, Command Line Inc.
 // SPDX-License-Identifier: Apache-2.0
 
-@use "../../mixins.scss";
-
 .dir-table-container {
     display: flex;
     flex-direction: column;