fix: improve text contrast on HOME and DOWNLOADS nav links (#1609)

## Motivation

Follow-up to #1601 (downloads page contrast fix). The HOME and DOWNLOADS
navigation links in the top-right header use `text-exo-light-gray`
(`oklch(0.6 0 0)`) which is too dim against the dark header background.

## Changes

Changed both nav links in `HeaderNav.svelte` from `text-exo-light-gray`
to `text-white/70` for better visibility. Hover state
(`text-exo-yellow`) is unchanged.

## Why It Works

`text-white/70` provides noticeably better contrast against
`bg-exo-dark-gray` while still looking subdued relative to the yellow
accent color on hover. This is consistent with the approach used in
#1601.

## Test Plan

### Manual Testing
- Verified both links are clearly readable on the home page and
downloads page
- Hover state still transitions to yellow as expected

### Automated Testing
- Dashboard builds successfully

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

dashboard/src/lib/components/HeaderNav.svelte

@@ -95,7 +95,7 @@
     {#if showHome}
       <button
         onclick={handleHome}
-        class="text-sm text-exo-light-gray hover:text-exo-yellow transition-colors tracking-wider uppercase flex items-center gap-2 cursor-pointer"
+        class="text-sm text-white/70 hover:text-exo-yellow transition-colors tracking-wider uppercase flex items-center gap-2 cursor-pointer"
         title="Back to topology view"
       >
         <svg
@@ -116,7 +116,7 @@
     {/if}
     <a
       href="/#/downloads"
-      class="text-sm text-exo-light-gray hover:text-exo-yellow transition-colors tracking-wider uppercase flex items-center gap-2 cursor-pointer"
+      class="text-sm text-white/70 hover:text-exo-yellow transition-colors tracking-wider uppercase flex items-center gap-2 cursor-pointer"
       title="View downloads overview"
     >
       {#if downloadProgress}