feat(ui): UI/UX polish — layout, sidebar, button, theme improvements

- Fix global scrollbar overflow by removing h-svh from html element
- Add h-full overflow-hidden to html/body for proper app-like layout
- Fix default button variant: add shadow-sm and hover:bg-primary/90
- Update sidebar create-issue button to bg-background with shadow
- Add WorkspaceAvatar component and search/new-issue actions to sidebar header
- Improve theme provider with TooltipProvider wrapper
- Polish various page layouts, pickers, modals, and code block styling
- Clean up custom.css unused styles

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/components/markdown/CodeBlock.tsx

@@ -1,5 +1,7 @@
 import * as React from 'react'
 import { codeToHtml, bundledLanguages, type BundledLanguage } from 'shiki'
+import { Copy, Check } from "lucide-react"
+import { Button } from "@/components/ui/button"
 import { cn } from '@/lib/utils'
 
 export interface CodeBlockProps {
@@ -177,36 +179,19 @@ export function CodeBlock({
         <span className="text-muted-foreground font-medium uppercase tracking-wide">
           {resolvedLang !== 'text' ? resolvedLang : 'plain text'}
         </span>
-        <button
+        <Button
+          variant="ghost"
+          size="icon-xs"
           onClick={handleCopy}
           className="opacity-0 group-hover:opacity-100 transition-opacity text-muted-foreground hover:text-foreground"
           aria-label="Copy code"
         >
           {copied ? (
-            <svg
-              className="w-4 h-4 text-green-500"
-              fill="none"
-              viewBox="0 0 24 24"
-              stroke="currentColor"
-            >
-              <path
-                strokeLinecap="round"
-                strokeLinejoin="round"
-                strokeWidth={2}
-                d="M5 13l4 4L19 7"
-              />
-            </svg>
+            <Check className="size-3.5 text-success" />
           ) : (
-            <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
-              <path
-                strokeLinecap="round"
-                strokeLinejoin="round"
-                strokeWidth={2}
-                d="M8 16H6a2 2 0 01-2-2V6a2 2 0 012-2h8a2 2 0 012 2v2m-6 12h8a2 2 0 002-2v-8a2 2 0 00-2-2h-8a2 2 0 00-2 2v8a2 2 0 002 2z"
-              />
-            </svg>
+            <Copy className="size-3.5" />
           )}
-        </button>
+        </Button>
       </div>
 
       {/* Code content */}