Add 'All Repos' nav link back to homepage (#14)

* fix: use Tree.Root.String() for JS/CSS extraction to avoid html/template escaping

RenderJS() and RenderCSS() were running static assets through
html/template's Execute(), which HTML-escapes < to &lt; in the output.
This broke all for-loops and comparisons in main.js (8 instances),
causing a syntax error that prevented all D3 graphs from rendering.

Same root cause as the template.HTML → template.JS fix: the
text/template → html/template migration in 8f1470b.

* Add 'All Repos' link to site header navigation

Adds a link back to repos.supermodeltools.com as the first item
in the nav bar on every page. Styled with accent color and a
subtle right border to separate it from the in-repo nav links.

Author: jonathanpopham

templates/_header.html

@@ -8,6 +8,7 @@
       {{.Site.Name}}
     </a>
     <nav class="site-nav">
+      <a href="https://repos.supermodeltools.com/" class="nav-all-repos">&larr; All Repos</a>
       <a href="/node_type/index.html">By Type</a>
       <a href="/domain/index.html">Domains</a>
       <a href="/language/index.html">Languages</a>

templates/_styles.css

@@ -69,6 +69,8 @@ a:focus-visible, button:focus-visible { outline: 2px solid var(--accent-light);
 .site-nav { display: flex; gap: 16px; align-items: center; overflow-x: auto; -webkit-overflow-scrolling: touch; }
 .site-nav a { color: var(--text-muted); font-size: 14px; font-weight: 500; white-space: nowrap; }
 .site-nav a:hover { color: var(--text); text-decoration: none; }
+.nav-all-repos { color: var(--accent-light) !important; padding-right: 12px; margin-right: 4px; border-right: 1px solid var(--border); }
+.nav-all-repos:hover { color: var(--text) !important; }
 
 /* Footer */
 .site-footer {