fix(ui): theme token consistency, seek bar a11y, colour polish

- Add --color-success variable to all three themes
- Replace hard-coded amber/green Tailwind colours with CSS variables:
    Pro badge + discount badge: bg-amber-400/15 → bg-(--color-brand)/15
    Saved confirmation:         text-green-400 → text-(--color-success)
    Catalogue flash messages:   emerald/yellow → success/brand tokens
    Login hover border:         border-zinc-600 → border-(--color-brand)/50
- Seek bar in mini-player now has aria-label (player_seek_label)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/routes/profile/+page.svelte

@@ -288,7 +288,7 @@
 		<div class="flex items-center justify-between gap-3 flex-wrap">
 			<h2 class="text-lg font-semibold text-(--color-text)">{m.profile_subscription_heading()}</h2>
 			{#if data.isPro}
-				<span class="inline-flex items-center gap-1.5 px-2.5 py-1 rounded-full text-xs font-bold bg-amber-400/15 text-amber-400 border border-amber-400/30 tracking-wide uppercase">
+				<span class="inline-flex items-center gap-1.5 px-2.5 py-1 rounded-full text-xs font-bold bg-(--color-brand)/15 text-(--color-brand) border border-(--color-brand)/30 tracking-wide uppercase">
 					<svg class="w-3.5 h-3.5" fill="currentColor" viewBox="0 0 24 24">
 						<path d="M11.99 2C6.47 2 2 6.48 2 12s4.47 10 9.99 10C17.52 22 22 17.52 22 12S17.52 2 11.99 2zm4.24 16L12 15.45 7.77 18l1.12-4.81-3.73-3.23 4.92-.42L12 5l1.92 4.53 4.92.42-3.73 3.23L16.23 18z"/>
 					</svg>
@@ -339,7 +339,7 @@
 						class="inline-flex items-center gap-2 px-4 py-2.5 rounded-lg border border-(--color-brand) text-(--color-brand) font-semibold text-sm hover:bg-(--color-brand)/10 transition-colors"
 					>
 						{m.profile_upgrade_annual()}
-						<span class="text-xs font-bold px-1.5 py-0.5 rounded bg-amber-400/15 text-amber-400 border border-amber-400/30">–33%</span>
+						<span class="text-xs font-bold px-1.5 py-0.5 rounded bg-(--color-brand)/15 text-(--color-brand) border border-(--color-brand)/30">–33%</span>
 					</a>
 				</div>
 			</div>
@@ -416,7 +416,7 @@
 				{settingsSaving ? m.profile_saving() : m.profile_save_settings()}
 			</button>
 			{#if settingsSaved}
-				<span class="text-sm text-green-400">{m.profile_saved()}</span>
+				<span class="text-sm text-(--color-success)">{m.profile_saved()}</span>
 			{/if}
 		</div>
 	</section>
@@ -502,7 +502,7 @@
 				{settingsSaving ? m.profile_saving() : m.profile_save_settings()}
 			</button>
 			{#if settingsSaved}
-				<span class="text-sm text-green-400">{m.profile_saved()}</span>
+				<span class="text-sm text-(--color-success)">{m.profile_saved()}</span>
 			{/if}
 		</div>
 	</section>