[style](trx-frontend-http): system font, button states, scrollbars, phosphor theme

System font stack: replace bare 'sans-serif' with system-ui / -apple-system
/ BlinkMacSystemFont / Segoe UI chain — sharper rendering on all platforms
at zero extra load cost.

Button hover/active: add transition (100ms) + color-mix hover brightening
+ active depression (translateY 1px) to all buttons. Previously buttons had
zero visual feedback on interaction.

Scrollbar styling: thin (6px) custom scrollbars via ::-webkit-scrollbar and
scrollbar-width/color for Firefox. Thumb uses border-color tinted with the
accent on hover — matches each theme automatically via CSS variables.

Phosphor theme: classic green-phosphor CRT aesthetic — near-black background,
#39ff14 neon-green accent, glow text-shadow on the freq display, matching
spectrum/waterfall canvas palette. Both dark and light variants included.
Registered in the style picker select, setStyle() valid list, and
CANVAS_PALETTE.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -51,6 +51,7 @@
             <option value="golden-rain">Donald</option>
             <option value="amber">Amber</option>
             <option value="fire">Fire</option>
+            <option value="phosphor">Phosphor</option>
           </select>
         </div>
         <button id="theme-toggle" class="header-bar-btn" type="button" aria-label="Toggle dark or light theme">Light</button>