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/popup/popup.css

@@ -14,6 +14,38 @@
   --cream:      #f4e9b7;
   --danger:     #c9685b;
   --radius:     6px;
+
+  /* First-run hint pulse colour (RGB tuple — alpha applied in keyframes). */
+  --pulse-rgb:  109, 191, 122;
+}
+
+/* ──────────────────────────────────────────────────────────────────────
+   Light mode (v0.5.0) — auto-follows OS theme via prefers-color-scheme.
+   Flips backgrounds + foregrounds to a cream-leaning palette while
+   preserving the brand green as the accent. Every existing rule that
+   uses the var(--*) tokens keeps working unchanged.
+   ────────────────────────────────────────────────────────────────────── */
+@media (prefers-color-scheme: light) {
+  :root {
+    --bg:         #f6f4ed;    /* cream-leaning off-white */
+    --bg-soft:    #ece5d2;    /* slightly darker for header bars */
+    --bg-row:     #ddd6c0;    /* hover background */
+    --bg-row-hi:  #c8c0a8;    /* active row background */
+    --fg:         #2a2f28;    /* dark forest-green-leaning text */
+    --fg-muted:   #6a7064;    /* muted secondary text */
+    --accent:     #2a7d3e;    /* darker brand green for primary contrast */
+    --accent-dim: #6dbf7a;    /* lighter brand green for fills */
+    --cream:      #b8861a;    /* darker amber — readable on light bg */
+    --danger:     #b53a2b;    /* darker red — readable on light bg */
+  }
+
+  /* State pills — light-mode adjustments so playing/buffering states
+     stay readable. The "playing" green stays bright but text flips white;
+     "buffering" cream gets dark text since the cream is now amber. */
+  .tuner-state[data-state="playing"]   { color: #fff; }
+  .tuner-state[data-state="buffering"] { color: #fff; }
+  .tuner-state[data-state="error"]     { color: #fff; }
+  .btn-primary[aria-pressed="true"]    { color: #fff; }
 }
 
 * { box-sizing: border-box; }