Tray icon: animated flame reflecting live burn rate

[tangentlin]

Motivation

Burnbar's tray icon is currently static. Since the app is literally named for "burning" tokens, the menu-bar icon is the one surface visible without opening anything — animating it ambiently would make the app's identity felt at a glance, not just readable in a menu.

Proposal

Pre-render a small set of flame frames (ember glow → roaring fire) and cycle them via tray.setImage() on a short interval (~1-2s), modulated by burn rate (tokens/min or $/hr over a short recent window) rather than absolute spend:

• Idle / no recent activity → gentle ember glow, barely animated (battery-friendly).
• Active heavy session → faster flicker / taller flame.

Feasibility notes

• TrayManager already swaps NativeImages (refreshIcon, dashboardIcon, the transparent spacer) — swapping the main tray icon on an interval is the same primitive, just applied to Tray.setImage().
• Needs a new lightweight burn rate derivation (short rolling window) distinct from today's cumulative totals — likely a small addition near capture-service.ts/derive.ts, not the existing daily/30d figures.
• Must stay cheap: template images are small (~22pt), and the interval should back off to a slow/static frame when burn rate is ~0 to avoid needless CPU/battery use.

Acceptance criteria

• A handful of flame frames render as monochrome template images (matches existing icon behavior, tints correctly in light/dark menu bars).
• Animation speed/intensity is driven by a short-window burn-rate metric, not cumulative cost.
• Idle state settles into a low-frequency (or static) frame — no continuous redraw when there's nothing to show.
• No measurable added CPU/battery burden when idle.