feat: mobile-first redesign and layout improvements

## Major Changes

### Mobile-First Responsive Design
- Converted entire app to mobile-first single-column layout
- Constrained max-width to 448px (mobile phone width)
- Black margins on desktop, centered content
- Removed all multi-column grids (md:grid-cols-3)

### Header Reorganization (3-Row Layout)
- Row 1: App logo + title + version
- Row 2: Security badges + action buttons (Empty, Reset)
- Row 3: Navigation tabs (Create, Backup, Restore, Blender)
- Replaced text buttons with emoji icons (📋 clipboard, 🙈 privacy mask)
- Consistent button sizing across all tabs

### Font Size Reductions
- Reduced all button text sizes for mobile density
- Main buttons: py-4 → py-3, added text-sm
- Labels: text-xs → text-[10px]
- Placeholders: consistent text-[10px] across all inputs
- Input fields: text-sm → text-xs, p-4 → p-3

### Create Tab Improvements
- Changed "GENERATE NEW SEED" from button-style to banner
- Left-aligned banner with gradient background
- Equal-width button grid (12/24 Words, Backup/Seed Blender)
- Used grid-cols-2 for consistent sizing

### Backup Tab Improvements
- Simplified drag-drop area with 📎 emoji
- Reduced padding and text sizes
- Cleaner, shorter copy
- PGP label font size: text-xs → text-[12px]

### SeedBlender Component
- Reorganized mnemonic input cards: textarea on row 1, buttons on row 2
- QR button (left) and X button (right) alignment
- Consistent placeholder text sizing (text-[10px])
- Shortened dice roll placeholder text

### HTTPS Development Server
- Added @vitejs/plugin-basic-ssl for HTTPS in dev mode
- Configured server to listen on 0.0.0.0:5173
- Fixed Web Crypto API issues on mobile (requires secure context)
- Enables testing on iPhone via local network

## Technical Details
- All changes maintain cyberpunk theme and color scheme
- Improved mobile usability and visual consistency
- No functionality changes, pure UI/UX improvements

src/index.css

@@ -2,6 +2,24 @@
 @tailwind components;
 @tailwind utilities;
 
+/* Mobile-first: constrain to phone width on all devices */
+#root {
+  max-width: 448px;
+  /* max-w-md = 28rem = 448px */
+  margin: 0 auto;
+  background: black;
+}
+
+body {
+  background: black;
+  overflow-x: hidden;
+}
+
+/* Ensure all content respects mobile width */
+* {
+  max-width: 100%;
+}
+
 body {
   margin: 0;
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',