feat: add 5 new themes via single QSS template engine

Replaces the two hard-coded .qss files with a single
resources/style_template.qss that uses @{token} placeholders.
apply_qss() now does regex substitution at runtime, so adding a
theme only requires a palette dict — no duplicate stylesheets.

New themes (Preferences → Color theme):
  • One Dark Pro   — VS Code classic dark
  • Nord           — arctic blue-gray dark
  • Tokyo Night    — deep purple-dark
  • Dracula        — classic purple-dark
  • GitHub Light   — clean minimal light

Existing Catppuccin Frappé (dark) and Latte (light) are unchanged.
Ctrl+Shift+T quick-toggle now recognises all dark themes via is_dark().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/ui/preferences_dialog.py

@@ -10,8 +10,15 @@ from app.config.settings import get_settings
 _FONT_FAMILIES = ["Consolas", "JetBrains Mono", "Fira Code", "Cascadia Code", "Courier New"]
 _PAGE_SIZES    = [50, 100, 500, 1000, 0]   # 0 → All
 _PAGE_LABELS   = ["50", "100", "500", "1 000", "All"]
-_THEMES        = [("dark",  "🌙  Dark  (Catppuccin Frappé)"),
-                  ("light", "☀️  Light (Catppuccin Latte)")]
+_THEMES = [
+    ("dark",         "🌙  Catppuccin Frappé   (dark blue-gray)"),
+    ("one_dark",     "⬛  One Dark Pro         (VS Code classic)"),
+    ("nord",         "❄️  Nord                 (arctic blue-gray)"),
+    ("tokyo_night",  "🌃  Tokyo Night          (deep purple-dark)"),
+    ("dracula",      "🧛  Dracula              (classic purple-dark)"),
+    ("light",        "☀️  Catppuccin Latte     (warm light)"),
+    ("github_light", "📄  GitHub Light         (clean minimal)"),
+]
 
 
 class PreferencesDialog(QDialog):
@@ -20,8 +27,8 @@ class PreferencesDialog(QDialog):
     def __init__(self, parent=None):
         super().__init__(parent)
         self.setWindowTitle("Preferences")
-        self.setMinimumWidth(400)
-        self.resize(420, 380)
+        self.setMinimumWidth(440)
+        self.resize(460, 400)
         self._build_ui()
         self._load()