feat(scene): wire borders + background colors and apply a design-mock palette

[esengine]

Closes the visual gap between the structural three-pane layout from
#1003 and the user's design mock. Previously every cell defaulted to
the terminal's stock bg/fg; now the trace frame honors a small
palette derived from the mock's oklch tokens, panes carry rounded
borders, and title / status rows sit on a panel-tinted bar.

Schema + Rust render

• `BoxLayout.background?: Color` — new field, same shape as
  `borderColor`. Schema added in both TS and Rust.
• `render_box` now runs `apply_decoration(layout, buf, area)` before
  laying out children:
  • if `background` is set, fills every cell of the box area with
    that bg (via ratatui Block's style).
  • if `borderStyle` is set, draws ratatui's box-drawing perimeter
    (Plain / Double / Rounded / Thick) in `borderColor`, shrinking
    inner area by 1 cell on each side for children.
  • both are independent — bg without border (title/status bars)
    works, border without bg works too.

ratatui's `Block` widget already does the right thing for the
combined case (fill then draw border on top), so we lean on that
rather than rolling perimeter-drawing by hand.

Design palette (`src/cli/ui/scene/theme.ts`)

oklch tokens from the mock converted to hex once, frozen as
`PALETTE.{bg, bg2, panel, card, border, borderStrong, fg, fg2,
muted, accent, accentStrong, success, warning, danger, violet}`.
Single source of truth — components don't pick raw colors anymore.

Producer side

• Outer frame: `background: PALETTE.bg` (dark blue-grey base, sits
  behind every pane).
• Title row: `background: PALETTE.panel`, brand prefix glyph in
  `accent`, model name in `violet` (matches the mock's gradient
  brand mark — actual gradient isn't doable in a terminal).
• Sidebar + ctx pane: `borderStyle: round` + `borderColor: border`
  • `background: bg2`. Reads as two soft cards flanking the main
    pane, same shape as the mock's panel/card style.
• Status bar: `background: PALETTE.panel`, busy/idle in
  warning/success hexes from the palette, wallet segment in
  success when present.
• Section headers (`SESSIONS`, `CONTEXT`) drop the dash decoration
  and use bold accent — cleaner now that bordered panes provide the
  framing.

Tests

• Rust (6 new): box-drawing chars at perimeter for Single / Round /
  Double border styles, border color applied to perimeter cells,
  background fills every cell, border shrinks inner area by 1 on
  each side.
• JS: existing 61 tests adjusted to the new shapes (status row is
  now always a box for its bg tint, busy/idle assert hex colors
  via PALETTE rather than named "green"/"yellow"). 2 new shape
  assertions: title/status carry a background; side/ctx panes
  carry borderStyle=round + background.

`cargo test -p reasonix-render --test render` → 23/23.
`npm run verify` → green via prepush gate.

What's still aspirational

Real CSS gradients, drop shadows, border-radius smoothness, custom
fonts, hover states — ratatui can't do any of these. That's the
floor for "TUI-native faithful to the mock"; everything above is
what we ship.

Refs #868