Keep timer digits evenly spaced

[PurpleSentinel]

Summary

• render countdown and elapsed-time digits in equal-width cells
• keep timer origins fixed as displayed figures change
• preserve proportional spacing for non-timer interface text
• document the timer typography behavior

Why

The native Montserrat glyphs have proportional digit widths, so recentering each updated time value caused the countdown to shift slightly. Tabular digit cells remove that movement while retaining the existing font and centering behavior.

Validation

• python -m unittest discover -s tests -v — 71 tests passed
• verified 11:11 and 88:88 use identical origins and widths on the connected RP2040 display at both timer sizes
• visually checked the hardware framebuffer for clean, unclipped rendering