feat(v0.5.0-prep): light mode support across all three surfaces

Implements the v0.4.0-plan-noted light mode using `@media
(prefers-color-scheme: light)` blocks that override the :root CSS
variables. The extension now auto-follows the user's OS theme —
no toggle, no setting, no preference UI required (Phase 2 toggle
deferred to a later release if anyone asks for it).

Light-mode palette preserves brand identity:
  --bg          #f6f4ed   cream-leaning off-white
  --bg-soft     #ece5d2   header / footer bars
  --bg-row      #ddd6c0   hover backgrounds
  --bg-row-hi   #c8c0a8   active rows
  --fg          #2a2f28   dark forest-green-leaning text
  --fg-muted    #6a7064   secondary text
  --accent      #2a7d3e   darker brand green (primary on light)
  --accent-dim  #6dbf7a   lighter brand green (secondary)
  --cream       #b8861a   darker amber — readable on light bg
  --danger      #b53a2b   darker red — readable on light bg

Every existing CSS rule using var(--*) tokens carries over unchanged.
The only surface-specific overrides are:

- State pills (popup + newtab) — text colour flips to white when
  the background becomes solid accent/cream/danger
- Primary play button (pressed state) — text colour flips
- Helmet watermark on NewTab — opacity bumped 0.025 → 0.04 via a
  new --watermark-opacity var since the helmet contrasts differently
  against the lighter background
- Toast (Options) — text colour flips to white on solid bg
- Danger button hover (Options) — text colour flips to white

Total: 3 files, +98 lines, 1 line modified.

Bundled with the v0.4.1-prep back-link fix already merged into this
branch. Together: a complete UX polish release (first-run hint shipped
in v0.4.0 plus back-link plus light mode = v0.5.0).

src/options/options.css

@@ -16,6 +16,34 @@
   --radius:     6px;
 }
 
+/* ──────────────────────────────────────────────────────────────────────
+   Light mode (v0.5.0) — auto-follows OS theme. Same approach as popup
+   + newtab: flip the :root vars only. Existing rules using var(--*)
+   carry over unchanged. Toast text + danger-button hover adjusted for
+   light-bg readability.
+   ────────────────────────────────────────────────────────────────────── */
+@media (prefers-color-scheme: light) {
+  :root {
+    --bg:         #f6f4ed;
+    --bg-soft:    #ece5d2;
+    --bg-row:     #ddd6c0;
+    --bg-row-hi:  #c8c0a8;
+    --fg:         #2a2f28;
+    --fg-muted:   #6a7064;
+    --accent:     #2a7d3e;
+    --accent-dim: #6dbf7a;
+    --cream:      #b8861a;
+    --danger:     #b53a2b;
+  }
+
+  /* Toast — keep readable when its background is solid accent / danger */
+  .opt-toast                    { color: #fff; }
+  .opt-toast[data-tone="error"] { color: #fff; }
+
+  /* Danger button hover — text flips to white when bg goes red */
+  .opt-btn--danger:hover { color: #fff; }
+}
+
 * { box-sizing: border-box; }
 
 html, body {