fix: remove hardcoded accent color values from theme-semantic.css

The accent color HSL components (--t-accent-h, --t-accent-s, --t-accent-l) were hardcoded in :root, which prevented the dynamically generated values from the CSS generator from taking effect properly.

Now the HSL components are only set by the CSS generator in .preview-frame, and theme-semantic.css only defines the derived colors that use those variables.

Also wrapped accent color inputs in a form element as required by Phoenix LiveView for phx-change events.

Verified that changing presets properly updates button colors (e.g., studio preset → blue buttons).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

assets/css/theme-semantic.css

@@ -4,10 +4,8 @@
    ======================================== */
 
 :root {
-  /* Accent color (dynamic, set by theme) */
-  --t-accent-h: 24;
-  --t-accent-s: 95%;
-  --t-accent-l: 53%;
+  /* Accent color - HSL components set dynamically by CSS generator */
+  /* Derived accent colors use the dynamic HSL values */
   --t-accent: hsl(var(--t-accent-h) var(--t-accent-s) var(--t-accent-l));
   --t-accent-hover: hsl(var(--t-accent-h) var(--t-accent-s) calc(var(--t-accent-l) - 8%));
   --t-accent-subtle: hsl(var(--t-accent-h) 40% 95%);