feat(01-02): welcome screen UI with onboarding flow

- Implement initWelcomeScreen with companion card rendering, layout toggle, start button
- Update index.html with welcome screen structure (companion-grid, layout-toggle, start-btn)
- Add CSS for companion cards, layout buttons, start button with hover/selected/disabled states
- Selecting a companion enables start, clicking start saves progress to IndexedDB and navigates to forest

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

src/styles/main.css

@@ -55,3 +55,109 @@ h3 {
 	opacity: 1;
 	min-height: 100vh;
 }
+
+/* Welcome screen */
+.welcome {
+	display: flex;
+	flex-direction: column;
+	align-items: center;
+	padding: 2rem;
+	max-width: 600px;
+	margin: 0 auto;
+}
+
+.welcome h1 {
+	font-family: "Quicksand", sans-serif;
+	font-size: 2rem;
+	color: var(--text-dark);
+}
+
+.welcome__subtitle,
+.welcome__layout-label {
+	font-size: 1.1rem;
+	color: var(--text-warm);
+	margin-bottom: 0.5rem;
+}
+
+.welcome__companions {
+	display: grid;
+	grid-template-columns: repeat(2, 1fr);
+	gap: 1rem;
+	width: 100%;
+}
+
+.companion-card {
+	background: white;
+	border: 3px solid transparent;
+	border-radius: 16px;
+	padding: 1.5rem;
+	cursor: pointer;
+	transition: border-color 0.2s;
+	text-align: center;
+	display: flex;
+	flex-direction: column;
+	align-items: center;
+	gap: 0.5rem;
+}
+
+.companion-card:hover {
+	border-color: var(--accent-gold);
+}
+
+.companion-card--selected {
+	border-color: var(--accent-green);
+	background: var(--bg-forest);
+}
+
+.companion-card__emoji {
+	font-size: 3rem;
+}
+
+.companion-card__name {
+	font-family: "Quicksand", sans-serif;
+	font-weight: 600;
+	font-size: 1.2rem;
+	color: var(--text-dark);
+}
+
+.welcome__layout-toggle {
+	display: flex;
+	gap: 0.5rem;
+	margin-top: 0.5rem;
+}
+
+.layout-btn {
+	padding: 0.5rem 1.5rem;
+	border-radius: 8px;
+	border: 2px solid var(--text-light);
+	background: white;
+	cursor: pointer;
+	font-family: "Nunito", sans-serif;
+	font-size: 1rem;
+}
+
+.layout-btn--active {
+	background: var(--accent-green);
+	color: white;
+	border-color: var(--accent-green);
+}
+
+.welcome__start-btn {
+	margin-top: 1.5rem;
+	padding: 1rem 3rem;
+	border-radius: 12px;
+	background: var(--accent-green);
+	color: white;
+	border: none;
+	font-family: "Quicksand", sans-serif;
+	font-weight: 700;
+	font-size: 1.3rem;
+	cursor: pointer;
+	opacity: 1;
+	transition: opacity 0.2s;
+}
+
+.welcome__start-btn:disabled {
+	opacity: 0.4;
+	cursor: not-allowed;
+}