perf: reduce per-frame allocations and fix memory leaks

[genebean]

I really like this card but the bugs were bothering me so I got Claude Code to help me make things better. This is the first of a set of 3 PRs that when combined are what I am now using at home.

• Cache computed theme colors in set hass so drawing functions do not call getComputedStyle on every animation frame
• Bind animation callback once (_boundDrawClock) instead of allocating a new closure on every requestAnimationFrame call
• Replace manual ctx.rotate undo math with ctx.save/ctx.restore in all drawing functions, eliminating floating-point drift risk
• Replace DOM div creation in drawNumbers with ctx.measureText for font height, eliminating forced reflow on every frame
• Fix clearRect: was only clearing top-left quadrant; now clears full canvas (canvas.width/2, canvas.height/2 → canvas.width, canvas.height)
• Fix CSS property name typo: '--primary-background-color:' → without ':'
• Store ResizeObserver as instance variable and add disconnectedCallback to disconnect it (and cancel pending rAF/timeout) when card is removed
• Move resize debounce timeout to per-instance (_resizeTimeout) so multiple clock cards on the same dashboard don't clobber each other's timers; remove module-level resizeTimeout/resizeDelay/debouncedOnResize

No behavior changes — VERSION remains 0.9.0.