feat(ui): swap the in-turn loader for a brain-wave sweep left of the send button

Phase 117 (owner request, live-mockup-confirmed): the phase-109 3-dot
cue becomes a compact ECG trace (49px, P/QRS/T) with a brand sweep
traveling the path (bwdraw, 42/140 dash segment, 0.9s loop), the
loader repositioned left of the button so its appearance never shifts
it. setUiState stays the sole owner of the loader's hidden attribute;
the reduced-motion variant stills the sweep. Unit + lifecycle-E2E pins
updated for the new contract.

frontend/assets/styles.css

@@ -1255,55 +1255,75 @@ details.thinking .thinking-text ul { margin: 0 0 0.5rem; }
   details.thinking summary::before { transition: none; }
 }
 
-/* Phase 109 (TODO.md L3, D16): the persistent in-turn loader — the
-   constant progress cue in the composer's status row, visible for the
-   ENTIRE active turn (send → terminal frame). Its hidden attribute is
-   owned solely by app.js's setUiState (shown iff inFlight — hidden by
-   construction in every terminal state, PLAN §7.4). DECORATIVE
-   (aria-hidden in index.html): #send-status carries the state's
-   meaning, so contrast is N/A here (§7.2 "text + color, never color
-   alone" — the state TEXT stays in #send-status). Compact horizontal
-   three-dot indicator reusing the EXISTING typing-dot animation (the
-   `typing` keyframes above — no new animation family), sized down for
-   the status row (4px dots, 3px gap): the element itself is the
-   middle dot (0.15s delay, like .typing span:nth-child(2)),
-   ::before/::after the outer two. */
+/* Phase 117 (owner request 2026-09-15, live-mockup-confirmed — the
+   phase-109 D16 dot cue replaced by a BRAIN-WAVE; the owner picked the
+   ECG over the first equalizer mock): a compact trace (P bump, QRS
+   spike, T bump — 49px wide, 8px flat tails; the middle is never
+   rescaled) in index.html as a static SVG. .bw-ghost is the
+   always-visible dim trace (the shape reads even when the sweep is
+   between peaks); .bw-pulse is the same path in brand with a
+   42/140 stroke-dash segment that TRAVELS the path — the classic
+   heart-monitor draw (the bwdraw keyframe below). Still the phase-109
+   D16 contract: visible for the ENTIRE active turn (send → terminal
+   frame), its hidden attribute owned solely by app.js's setUiState
+   (shown iff inFlight — hidden by construction in every terminal
+   state, PLAN §7.4). DECORATIVE (aria-hidden in index.html):
+   #send-status carries the state's meaning, so contrast is N/A here
+   (§7.2 "text + color, never color alone" — the state TEXT stays in
+   #send-status). The container is 44px = .send-btn min-height (house
+   cross-reference — keep in lockstep), the trace centered against the
+   button. The NEW bwdraw family is deliberate: the phase-109 "reuse
+   the typing animation" pin is superseded by the owner's redesign.
+   The loader sits LEFT of the button in the markup (index.html) so
+   its appearance/disappearance never shifts the button. */
 .turn-loader {
   display: inline-flex;
-  align-items: center;
-  gap: 3px;
-  width: 4px;
-  height: 4px;
-  border-radius: 50%;
-  background: var(--ink-soft);
-  opacity: 0.5;
-  align-self: center; /* the composer row is flex-end aligned — center the cue */
-  animation: typing 1.2s infinite ease-in-out;
-  animation-delay: 0.15s;
+  align-items: center; /* center the trace against the button */
+  width: auto;
+  height: 44px; /* = .send-btn min-height (44px) — keep in lockstep */
+  background: none;
+  border-radius: 0;
+  opacity: 1;
+  align-self: flex-end; /* the composer row is flex-end aligned */
+  animation: none; /* the sweep carries the motion, not the container */
 }
-.turn-loader::before,
-.turn-loader::after {
-  content: "";
-  width: 4px;
-  height: 4px;
-  border-radius: 50%;
-  background: var(--ink-soft);
-  opacity: 0.5;
-  animation: typing 1.2s infinite ease-in-out;
+.turn-loader svg {
+  width: 49px;
+  height: 32px;
+  overflow: visible; /* the QRS spike (y=3) may exceed the box top */
 }
-.turn-loader::before { animation-delay: 0s; }
-.turn-loader::after { animation-delay: 0.3s; }
-/* Phase 109 (TODO.md L3, D16): reduced motion — the loader mirrors the
-   typing dots' treatment (static dots, no pulse — §7.2 house law).
-   This override MUST sit AFTER the main .turn-loader rule above:
-   equal specificity, and the later rule wins the cascade — a media
-   block placed before the main rule would lose to the `animation:
-   typing …` declaration and the dots would keep pulsing under
-   reduced motion (the phase-109 task-03 E2E caught it). */
+.turn-loader .bw-ghost {
+  fill: none;
+  stroke: var(--ink-soft);
+  stroke-opacity: 0.3; /* the dim always-visible trace */
+  stroke-width: 2;
+}
+.turn-loader .bw-pulse {
+  fill: none;
+  stroke: var(--brand);
+  stroke-width: 2.5;
+  stroke-linecap: round;
+  stroke-linejoin: round;
+  /* The sweep: a 42/140 bright segment on the pathLength-normalized
+     path — the dash cycle (42 + 98) equals the normalized length, so
+     the bwdraw loop is seamless (no visible restart point). */
+  stroke-dasharray: 42 98;
+  animation: bwdraw 0.9s linear infinite;
+}
+@keyframes bwdraw {
+  from { stroke-dashoffset: 140; }
+  to { stroke-dashoffset: 0; }
+}
+/* Phase 117: reduced motion — the sweep is STILLED (the full trace
+   shows static, no travel — §7.2 house law), never HIDDEN: visibility
+   is owned by the JS hidden attribute in setUiState (D16), not by
+   CSS. This override MUST sit AFTER the main rules above: equal
+   specificity, and the later rule wins the cascade — a media block
+   placed before the main rule would lose the cascade and the sweep
+   would keep traveling under reduced motion (the phase-109 cascade
+   pitfall, carried over). */
 @media (prefers-reduced-motion: reduce) {
-  .turn-loader,
-  .turn-loader::before,
-  .turn-loader::after { animation: none; opacity: 0.7; }
+  .turn-loader .bw-pulse { animation: none; stroke-dasharray: none; }
 }
 
 /* ---------- Empty state & suggestions ---------- */