fix(ui): remove side-stripe borders in favour of subtle bg tints

Side-stripe borders (border-left: 2px or 3px in an accent colour, with
the rest of the element having no border) are the textbook AI-coded-IDE
"selected-state" pattern. The brand vocabulary is colour-tint and
chevron, not stripe-and-fill.

Replaced eight sites:
- VirtualSegmentList active/match/default segment rows: bg-accent/10,
  bg-warning/10, hover:bg-hover (drop the stripes; bg-warning bumped
  /5 → /10 so it's visible without the stripe doing the work).
- viewer/+page.svelte: same pattern as VirtualSegmentList.
- TasksPage profile-list tabs: bg-accent/10 + font-medium for active,
  hover:bg-hover for inactive. rounded-md added so the tint follows
  the row outline.
- ToastViewport: replaced border-left + variant colour with full 1px
  border + bg tint + border-color tint (color-mix() with the matching
  semantic token at 35% / 10%). Also removed the orphan --moss /
  --signal / --ember tokens — they were not defined in app.css and
  fell back to hex literals.
- preview/+page.svelte: dropped the phase-coloured left stripe and
  the borderColorClass derivation; the header already has a pulsing
  dot / animated bars / spinner for each phase, so the stripe was
  redundant.

src/routes/preview/+page.svelte

@@ -183,16 +183,10 @@
       : phase === "cleanup" ? "Cleaning up"
       : "Final",
   );
-  let borderColorClass = $derived(
-    phase === "final" ? "border-success"
-      : phase === "cleanup" ? "border-accent"
-      : "border-border",
-  );
 </script>
 
 <div
-  class="h-full w-full flex flex-col bg-bg text-text p-3 gap-2 border-l-2 {borderColorClass}"
-  style="transition: border-color var(--duration-ui) ease-out"
+  class="h-full w-full flex flex-col bg-bg text-text p-3 gap-2"
   data-tauri-drag-region
 >
   <header class="flex items-center justify-between gap-2 text-[11px] text-text-secondary" data-tauri-drag-region>

src/routes/viewer/+page.svelte

@@ -522,10 +522,10 @@
               bind:this={segmentRefs[seg._idx]}
               class="group flex gap-3 px-3 py-2 rounded-lg cursor-pointer transition-colors
                 {activeSegmentIdx === seg._idx
-                  ? 'bg-accent/10 border-l-2 border-accent'
+                  ? 'bg-accent/10'
                   : matchingSegments.has(seg._idx)
-                    ? 'bg-warning/5 border-l-2 border-warning/40'
-                    : 'border-l-2 border-transparent hover:bg-hover'}"
+                    ? 'bg-warning/10'
+                    : 'hover:bg-hover'}"
               onclick={() => seekToTime(seg.start)}
               ondblclick={() => startEditing(seg._idx)}
               role="button"