fix: warnings and icon rendering

- Remove unused imports (HashMap, TextEdit)
- Remove unnecessary mut on fonts
- Replace ✕ (U+2715) with × (U+00D7) for close/cross icons
- Replace ＋ (U+FF0B) with + (U+002B) for New Connection button
- Replace ▲/▼ (U+25B2/U+25BC) with ↑/↓ (U+2191/U+2193) for sort arrows

src/ui/mod.rs

@@ -3,3 +3,4 @@ pub mod dialogs;
 pub mod drag;
 pub mod panels;
 pub mod state;
+pub mod theme;