fix: add missing var() wrappers to CSS custom properties (#74)

Six CSS declarations used bare custom property names (e.g.
`color: --pg-text`) instead of the required `var()` function
(e.g. `color: var(--pg-text)`). Browsers ignore invalid values,
so these styles were silently not applied.

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: dobby-coder[bot]

src/lib/components/Header.svelte

@@ -129,7 +129,7 @@
 
     &.selected a {
       text-decoration: 2px underline;
-      text-decoration-color: --pg-text;
+      text-decoration-color: var(--pg-text);
       text-underline-offset: 4px;
     }
 
@@ -144,7 +144,7 @@
         height: 2px;
         bottom: 0;
         left: 0;
-        background-color: --pg-text;
+        background-color: var(--pg-text);
         transform-origin: bottom right;
         transition: transform 0.25s ease-out;
       }

src/lib/components/LocaleSwitcher.svelte

@@ -130,7 +130,7 @@
 
   .selected .language-label {
     text-decoration: 2px underline;
-    text-decoration-color: --pg-text;
+    text-decoration-color: var(--pg-text);
     text-underline-offset: 4px;
   }
 

src/lib/components/filesharing/Done.svelte

@@ -79,7 +79,7 @@
     h2 {
         font-size: var(--pg-font-size-2xl);
         font-weight: var(--pg-font-weight-bold);
-        color: --pg-text;
+        color: var(--pg-text);
         margin: 0 0 1rem 0;
         text-align: center;
     }