feat(chat-bg): redesign 19 chat backgrounds as modular per-pattern files

Same treatment as the seasonal themes: split the 502-line chatBackground.ts
Record into one premium module per background under lotus/backgrounds/ (each
exposes a tuned dark + light ChatBgVariants), one Opus agent per background
against a shared brief. chatBackground.ts now assembles DARK/LIGHT from the
modules; getChatBg is unchanged. Carbon + Aurora are kept inline as-is (user
favorites); none stays the empty layer.

Every redesign: layered oklch palettes, seamless tiling with worked-out tile
math (integer-multiple periods; edge-wrapping inline-SVG data-URIs for
circuit/hexgrid/waves/herringbone/chevron/tactical), independently-tuned
dark+light (not a recolor), and low "felt-not-read" opacity so chat text stays
WCAG-AA legible. The 5 animated backgrounds (rain, star drift, grid pulse,
aurora flow, fireflies) each colocate a vanilla-extract keyframe .css.ts,
animate only background-position for a jump-free loop, and — since getChatBg
strips animation for reduced-motion — render a finished static frame too.

Redesigned: blueprint, stars, topographic, herringbone, crosshatch, chevron,
polka, triangles, plaid, tactical, circuit, hexgrid, waves, neon, anim-rain,
anim-stars, anim-pulse, anim-aurora, anim-fireflies.

Gates: tsc clean, ESLint clean, Prettier clean, build OK, 551 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/lotus/backgrounds/animRain.css.ts

@@ -0,0 +1,22 @@
+import { keyframes } from '@vanilla-extract/css';
+
+// Digital Rain — a slow vertical PAN of the streak columns.
+//
+// The streak SVG tile is authored 200px tall (see animRain.ts, backgroundSize
+// height = 200px). The falling illusion is a pure background-position translate
+// downward by EXACTLY one tile height (200px) over the cycle, so the loop is
+// perfectly seamless — the pixel at y re-enters where the pixel at y-200 was,
+// which is identical because the tile repeats.
+//
+// Only the first background layer (the streak SVG) is panned; every subsequent
+// comma-separated layer is kept at its authored position ('0 0') so the base
+// gradients / vignette stay put while the rain falls over them. Listing a value
+// per layer is required — a single value would pan ALL layers.
+//
+// getChatBg adds `willChange: 'background-position'` for the animated case, and
+// STRIPS this whole `animation` for reduced-motion, at which point the static
+// backgroundPosition authored in animRain.ts is what shows.
+export const rainFall = keyframes({
+  '0%': { backgroundPosition: '0 0, 0 0, 0 0, 0 0' },
+  '100%': { backgroundPosition: '0 200px, 0 0, 0 0, 0 0' },
+});