style(design): FINDING-001 — increase touch targets to 44px minimum

Hamburger button: 32x32 → 44x44px
Theme toggle: 36x36 → 44x44px
Sidebar nav links: add min-height: 44px

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

src/shared/styles/layout/header.css

@@ -71,12 +71,12 @@
   display: flex;
   flex-direction: column;
   justify-content: space-around;
-  width: 32px;
-  height: 32px;
+  width: 44px;
+  height: 44px;
   background: transparent;
   border: none;
   cursor: pointer;
-  padding: 4px;
+  padding: 10px;
   z-index: 10;
   transition: all 0.3s ease;
 }
@@ -169,8 +169,8 @@
   display: flex;
   align-items: center;
   justify-content: center;
-  width: 36px;
-  height: 36px;
+  width: 44px;
+  height: 44px;
   background: transparent;
   border: 1px solid var(--surface-border, #e2e8f0);
   border-radius: var(--radius-md, 6px);

src/shared/styles/layout/navigation.css

@@ -83,6 +83,7 @@
   align-items: center;
   gap: var(--space-sm, 8px);
   padding: var(--space-sm, 8px) var(--space-md, 12px);
+  min-height: 44px;
   color: var(--color-text, #111827);
   text-decoration: none;
   border-radius: var(--radius-md, 6px);