feat: light/dark theme support

Add a Theme preference (Automatic / Light / Dark). Automatic follows the
system org.gnome.desktop.interface color-scheme; the dark theme is
unchanged. Light styling lives in light.css, every rule scoped under a
.strata-theme-light class the panel toggles on its root box, loaded into the
St theme context by the extension. St's CSS engine has no custom properties,
so this specificity-based scoped override is how the palette is switched
without generating a stylesheet at runtime; switching is a single class
toggle off the ingest/render hot paths.

Also in this change:
- fix: the keyboard shortcut now hides the panel when it is already open.
  The binding lacked Shell.ActionMode.POPUP, so while the panel's modal grab
  was active the second press was swallowed and toggle() never ran.
- fix: the search placeholder ("Search...") is now legible in the light
  theme; it previously inherited a light-on-dark Shell color.
- pack: bundle light.css via --extra-source (gnome-extensions pack only
  auto-includes stylesheet.css), so packaged installs ship the light theme.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

strata@edu4rdshl.dev/schemas/org.gnome.shell.extensions.strata.gschema.xml

@@ -52,6 +52,17 @@
       <description>When enabled, clicking or pressing Enter on a history item moves it to position 1 in the list.</description>
     </key>
 
+    <key name="theme" type="s">
+      <choices>
+        <choice value="auto"/>
+        <choice value="light"/>
+        <choice value="dark"/>
+      </choices>
+      <default>'auto'</default>
+      <summary>Panel color theme</summary>
+      <description>Auto follows the system light/dark color scheme (org.gnome.desktop.interface color-scheme). Light and Dark force a fixed theme.</description>
+    </key>
+
     <key name="panel-position" type="s">
       <choices>
         <choice value="top-center"/>