Add progress-bar background with progressive color to free spots label in Upcoming Days

[Copilot]

The Upcoming Days view showed free-spot counts as a plain badge, requiring readers to parse numbers to gauge availability. This adds a progress-bar fill behind the label, sized by the free/total ratio and colored progressively from green (all free) through yellow to red (none free).

Changes

• src/services/availability.ts (new) — Pure availabilityVisual(freeCount, totalSpots) returning { percent, color }. percent is the fill width; color is an HSL value interpolated across hue 0→120 (red→green). Guards against divide-by-zero and clamps out-of-range counts.
• src/routes/future/index.tsx — Badge exposes fill via --availability-percent / --availability-color inline CSS vars and gains role="progressbar" + aria-valuenow/min/max.
• src/global.css — .availability-badge becomes a positioned container; a ::before pseudo-element renders the fill from the CSS vars (behind text, with a transition).
• src/services/availability.test.ts (new) — Covers 0/50/100% fill, color mapping, zero-total, and clamping edges.

availabilityVisual(0, 10);  // { percent: 0,   color: "hsl(0, 65%, 45%)" }   red
availabilityVisual(5, 10);  // { percent: 50,  color: "hsl(60, 65%, 45%)" }  yellow
availabilityVisual(10, 10); // { percent: 100, color: "hsl(120, 65%, 45%)" } green

The discrete badge-full/badge-low/badge-available text-color classes are retained; the progressive fill sits behind the existing styling.

• Fixes Use a progress bar background on free spots label in Upcoming Days view #19