Gamification: budget "fuse" warning animation

[tangentlin]

Motivation

Several of the animated-visual ideas (tray flame, dashboard gauge) are purely decorative; this one turns the same visual language into a genuinely useful signal. If a user opts into a soft daily/monthly budget, the flame/gauge changing character (color, flicker speed) as they approach it gives an ambient early warning without needing alerts or notifications.

Proposal

Let the user optionally set a soft budget (daily and/or monthly $ threshold). As current spend approaches it, the tray flame animation (see: tray icon animated flame issue) and/or dashboard gauge (see: burn-rate gauge issue) shift color/intensity (calm → warm → hot) to reflect proximity — a literal "burning fuse."

Feasibility notes

• Depends on the tray-flame and/or dashboard-gauge features existing first (or being built alongside) — this issue is the "make it useful" layer on top of those, not a standalone visual.
• Budget value needs a small persisted setting, following the existing pattern in settings.ts (refresh interval is already persisted the same way).
• Non-goal per user's original brainstorm: no hard enforcement or blocking — purely an ambient visual signal, consistent with Burnbar's non-intrusive, tray-only philosophy (no popups/alerts).

Acceptance criteria

• Optional budget setting (daily and/or monthly), persisted like existing preferences, off by default.
• When set, tray flame and/or dashboard gauge visually shift as spend approaches the threshold.
• No enforcement, blocking, or interruptive notifications — visual signal only.
• With no budget set, behavior is unchanged from the base flame/gauge animation.