Add smooth timer colour gradient

[PurpleSentinel]

Summary

• replace abrupt 85% and 95% warning backgrounds with a continuous progress-based gradient
• move through green, yellow, amber, and red using evenly spaced session-percentage anchors
• use a deep-purple overrun state with white text
• choose black or white timer text automatically using WCAG-style contrast
• preserve once-per-displayed-second framebuffer updates and existing rest-session behaviour

Why

The previous discrete colour changes did not communicate progress smoothly and did not follow the requested traffic-light progression. The new renderer makes the background a continuous visual indication of elapsed session percentage for every configured duration while retaining strong timer readability.

Implementation

• interpolate in RGB888 with rounded integer arithmetic
• convert only the final colour to the GC9A01 framebuffer's byte-swapped RGB565 value
• derive colour from whole elapsed seconds so no second framebuffer or frame cache is required
• select whichever of black or white provides the higher contrast against each generated background

Impact

• fixed and Auto rotation, Launch Mode, stop gestures, timer digits, and saved settings are unchanged
• rest-session and Ready-screen colours remain unchanged
• normal full-frame transfers remain bounded to one per displayed second

Validation

• python -m unittest discover -s tests — 143 tests passed
• deployed to the connected Waveshare RP2040 under MicroPython 1.21.0
• clean application startup confirmed with saved settings restored and no traceback
• 601 gradient and contrast calculations completed on-device in 261,370 microseconds with no retained heap growth

Closes#66
Resolves#2