feat(ui): cozy polish — primitives, motion, empty states

Adds 7 reusable primitives in src/lib/components/ui/ and migrates ~70 hand-rolled
call sites across forms, admin panels, and routes. Tokens unchanged — same Cozy
Home palette, just consistently applied.

Primitives
- Switch: pill toggle, role=switch, terracotta track, cubic-bezier knob
- Button: 5 variants × 4 sizes, press-squash, loading spinner, buttonClass()
  helper for <a> link-as-CTA cases
- Checkbox: rounded square with animated check-draw + indeterminate
- Select: native <select> with Cozy chevron + matched radius
- Slider: gradient track, terracotta-bordered knob, aria-valuetext
- Input + Field: documented in CLAUDE.md for future use
- 9 buttonClass unit tests

Migrations
- 23 <input type=checkbox> → <Switch> (boolean settings)
- 5 multi-select checkboxes → <Checkbox> (DiscoveryPanel, sys-stats metrics)
- ~28 <select> → <Select>
- 17 <input type=range> → <Slider> (ThemeCustomizer's hue picker kept custom)
- ~25 hand-rolled buttons → <Button> / buttonClass()

Surface polish
- Admin section wrappers: rounded-lg → rounded-[1.4rem] + shadow-soft
  (resolves the Phase-5 tradeoff from the Cozy migration memo)
- BoardPropertiesPanel: live theme preview swatch showing computed hsl() on a
  sample button; hue/sat use Slider; bg/cardSize use Select
- AppHealthBadge: role=status + aria-live=polite; .status-degraded (slow
  amber breathing) and .status-offline (single attention flash) now applied
- AppForm collapse triggers: rotating chevron + aria-expanded
- Empty states for /boards and /apps: inline SVGs using --room-* tokens
  (peach/sky/sage/butter) instead of generic Lucide icons
- Login Remember Me: showcase Switch (first-impression surface)

Motion (src/app.css)
- New cozy-rise / cozy-rise-stagger for staggered grid reveals (/boards, /apps)
- New cozy-expand for accordion sections (healthcheck, integration, wallpaper)
- All motion respects prefers-reduced-motion

CLAUDE.md
- New project guide with a mandatory Frontend reuse table — every primitive
  documented with "never use raw <input type=checkbox>/<select>/<range>" and
  "do not repeat rounded-xl bg-primary px-4 py-2 ..." rules

Verification
- npm run check: 0 errors, 0 warnings, 5831 files
- npm test: 301 passing
- npm run lint: 0 errors (19 pre-existing warnings unchanged)
- npm run build: ✔ done

Branch is feat/cozy-polish, ready to PR against master.

src/routes/login/+page.svelte

@@ -3,6 +3,8 @@
 	import { superForm } from 'sveltekit-superforms';
 	import type { PageData } from './$types.js';
 	import AmbientBackground from '$lib/components/background/AmbientBackground.svelte';
+	import Switch from '$lib/components/ui/Switch.svelte';
+	import Button from '$lib/components/ui/Button.svelte';
 
 	let { data }: { data: PageData } = $props();
 
@@ -106,12 +108,12 @@
 				</div>
 
 				<div class="flex items-center justify-between gap-4">
-					<label class="flex items-center gap-2 text-sm text-muted-foreground">
-						<input
-							type="checkbox"
+					<label class="flex cursor-pointer items-center gap-3 text-sm text-muted-foreground">
+						<Switch
 							name="rememberMe"
 							bind:checked={$form.rememberMe}
-							class="h-4 w-4 rounded border-input text-primary focus-visible:ring-2 focus-visible:ring-primary/30"
+							size="sm"
+							ariaLabel={$t('auth.remember_me')}
 						/>
 						<span>{$t('auth.remember_me')}</span>
 					</label>
@@ -120,20 +122,9 @@
 					</a>
 				</div>
 
-				<button
-					type="submit"
-					disabled={$submitting}
-					class="w-full rounded-xl bg-primary px-4 py-2.5 text-sm font-semibold text-primary-foreground shadow-[var(--shadow-soft)] transition-all hover:-translate-y-0.5 hover:bg-primary/90 focus:outline-none focus-visible:ring-2 focus-visible:ring-primary/30 disabled:cursor-not-allowed disabled:opacity-50"
-				>
-					{#if $submitting}
-						<span class="flex items-center justify-center gap-2">
-							<span class="h-4 w-4 animate-spin rounded-full border-2 border-primary-foreground border-t-transparent"></span>
-							{$t('auth.login_submitting')}
-						</span>
-					{:else}
-						{$t('auth.login_submit')}
-					{/if}
-				</button>
+				<Button type="submit" size="lg" fullWidth disabled={$submitting} loading={$submitting}>
+					{$submitting ? $t('auth.login_submitting') : $t('auth.login_submit')}
+				</Button>
 			</form>
 		{/if}