Add white logo and revamp UI styles

Add a new white SVG logo asset and wire it into the welcome and sidebar views; overhaul theme and component styles for a cleaner, modern dark theme. Changes include:

- Added frontend/assets/logos/Gitpub-Word-Logo-2-White.svg and referenced it in frontend/js/app.js (welcome and sidebar).
- Updated CSS variables and base styles in frontend/css/base.css: adjusted colors, radii, shadow, typography, input focus, button spacing and variants (primary, primary-blue, danger), and transitions.
- Refined component styles in frontend/css/components.css: updated panel/sidebar backgrounds, paddings, repo-card sizing and hover, tab and pill behaviors, spacing, logo/sidebar-brand classes, git output font stack, and various UI polish tweaks (font sizes, weights, paddings, and responsive adjustments).

These changes standardize the dark theme, introduce visual improvements and spacing refinements, and add branding to the UI.

frontend/js/app.js

@@ -98,6 +98,10 @@ function welcomeView() {
   appRoot.innerHTML = `
     <div class="welcome-wrap">
       <div class="welcome-card stack">
+        <div class="welcome-logo">
+          <img src="/assets/logos/Gitpub-Word-Logo-2-White.svg" alt="Gitpub" class="welcome-logo-img" />
+          <span class="welcome-logo-desktop">Desktop</span>
+        </div>
         <div class="panel stack">
           <h1 class="title">Welcome to Gitpub Desktop</h1>
           <p class="subtitle">
@@ -170,7 +174,10 @@ function dashboardView() {
     <div class="layout">
       <aside class="sidebar stack">
         <div class="sidebar-brand">
-          <h2 class="title">Gitpub Desktop</h2>
+          <div class="sidebar-logo">
+            <img src="/assets/logos/Gitpub-Word-Logo-2-White.svg" alt="Gitpub" class="sidebar-logo-img" />
+            <span class="sidebar-logo-desktop">Desktop</span>
+          </div>
         </div>
 
         <div class="server-chip">