feat(ui): implement hover for switch (#9057)

Followup to https://codeberg.org/forgejo/forgejo/issues/6459

Preview:
- https://codeberg.org/attachments/cc12a227-c6eb-4ab5-91ad-0ef967f88c7a
- https://codeberg.org/attachments/a13c8cb6-f654-430c-a6c6-990a2a7f0911

Changes:
- hovered items now have background painted with transition
- added E2E testing: mostly for CSS logic introduced in this PR but also a bit for existing functionality
- variable --color-active is now consistently non-transparent: it already was in forgejo-dark but is now in all themes
- some code improvements in switch.css

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9057
Reviewed-by: Beowulf <beowulf@beocode.eu>

web_src/css/themes/theme-forgejo-light.css

@@ -240,7 +240,7 @@
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(6 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #0000001d;
   --color-hover: #e4e4e4aa;
-  --color-active: #d4d4d8aa;
+  --color-active: #e2e2e5;
   --color-menu: var(--zinc-100);
   --color-card: var(--zinc-50);
   --fancy-card-bg: var(--zinc-100);

web_src/css/themes/theme-gitea-dark.css

@@ -205,7 +205,7 @@
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(40 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #e8e8ff28;
   --color-hover: #e8e8ff19;
-  --color-active: #e8e8ff24;
+  --color-active: #393b43;
   --color-menu: #151a1e;
   --color-card: #151a1e;
   --fancy-card-bg: #14171a;

web_src/css/themes/theme-gitea-light.css

@@ -205,7 +205,7 @@
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(6 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #0000171d;
   --color-hover: #00001708;
-  --color-active: #00001714;
+  --color-active: #ebebed;
   --color-menu: #f8f9fb;
   --color-card: #f8f9fb;
   --fancy-card-bg: #ffffff;