feat(styles): restructure UI styles and add main composition

New Features
- Added styles/main.css aggregating imports and global defaults
- Renamed battery.css → battery-widget.css for widget-specific styling

Enhancements
- Added smooth transitions and hover effects in panel.css
- Improved OSD styling with rounded corners, translucent background, and better spacing
- Updated button margins, paddings, and border‑radius for consistency

Bug Fixes
- Fixed missing border‑radius and spacing in panel buttons
- Corrected workspace button active/urgent styling

styles/osd.css

@@ -0,0 +1,38 @@
+#osd {
+    background-color: alpha(#fff, 0.09);
+    padding: 12px 20px;
+    margin: 70px;
+    min-height: 200px;
+    border-radius: 16px;
+}
+
+#osd scale {
+    min-width: 180px;
+}
+
+#osd trough {
+    background: var(--surface);
+    min-height: 15px;
+    margin-right: 4px;
+    transition: all 0.25s cubic-bezier(0.175, 0.885, 0.32, 1.275);
+    border-radius: 16px;
+}
+
+#osd trough highlight {
+    border-radius: 100px;
+    background: var(--primary);
+    transition: all 0.25s cubic-bezier(0.175, 0.885, 0.32, 1.275);
+}
+
+#osd.muted trough highlight,
+#osd.muted slider,
+#osd scale.muted trough highlight,
+#osd scale.muted slider {
+    background-color: var(--surface-bright);
+}
+
+#brighntess-icons.muted,
+#vol-icon.muted,
+#mic-icon.muted {
+    color: var(--outline);
+}