[fix](trx-frontend-http): vendor DSEG14 font locally to avoid CDN content blockers

Same issue as Leaflet — content blockers block the jsdelivr CDN request,
causing the seven-segment font to fail loading and fall back to monospace.

Also replace preload-to-stylesheet swap with media="print" onload swap
for themes.css and leaflet.css to eliminate Safari preload warnings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -77,7 +77,7 @@
   font-style: normal;
   font-weight: 400;
   font-display: swap;
-  src: url('https://cdn.jsdelivr.net/npm/@fontsource/dseg14-classic/files/dseg14-classic-latin-400-normal.woff2') format('woff2');
+  src: url('/vendor/dseg14-classic-latin-400-normal.woff2') format('woff2');
   unicode-range: U+0030-0039, U+002E, U+002D, U+0020, U+002B;
 }