Add a smooth progress-based timer colour gradient

[PurpleSentinel]

Parent: #23

Summary

Replace the track timer's abrupt 85% and 95% background changes with a continuous, progress-based colour gradient. The display should communicate the whole session at a glance, progressing from green through yellow and amber to red as scheduled time expires, then using deep purple for overrun.

User experience

• A track session starts with a green background.
• As time elapses, the background fades smoothly from green to yellow, yellow to amber, and amber to red.
• The colour progression is based on elapsed percentage, so it behaves consistently for every configured track-session duration.
• The scheduled-time colour anchors are evenly distributed: green at 0%, yellow at one-third, amber at two-thirds, and red at 100%.
• Once scheduled time has expired, the timer remains at 00:00 with a deep-purple background and white text while elapsed time continues to show the overrun.
• Countdown and elapsed-time text must remain highly visible throughout every intermediate colour, including the exact anchor colours and overrun state.

Functional behavior

Scheduled session gradient

• Calculate progress as elapsed scheduled time divided by total scheduled duration and clamp it to 0–100%.
• Linearly interpolate the RGB colour inside the current green/yellow, yellow/amber, or amber/red segment.
• Derive gradient state from the displayed elapsed second so the background and timer digits update together without unnecessary full-frame redraws.
• Preserve the current 00:00 remaining-time behavior at and after expiry.

Text contrast

• Evaluate each generated background colour and automatically select black or white text, choosing whichever provides the stronger WCAG-style contrast.
• Apply the selected colour to both the large remaining time and smaller elapsed time.
• Do not rely on fixed text-colour thresholds tied only to the named anchor colours.

Overrun

• At scheduled expiry, enter the semantically distinct deep-purple overrun state.
• Keep the existing double-tap stop behavior and elapsed overrun counter unchanged.

Hardware, safety, and compatibility

• Generate native RGB565 values compatible with the Waveshare 240×240 LCD.
• Do not allocate a second framebuffer or retain a full set of pre-rendered gradient frames.
• Keep display updates bounded to visible timer changes so SPI traffic, touch responsiveness, and RP2040 heap usage remain safe.
• Track/rest duration settings, Launch Mode, fixed/Auto rotation, stop gestures, and saved configuration must remain compatible.
• The rest timer and Ready screen are outside this feature and retain their current colour behavior.

Acceptance criteria

• Track background is green at 0%, yellow at one-third, amber at two-thirds, and red immediately before scheduled expiry.
• Intermediate scheduled-session frames use smoothly interpolated colours without abrupt 85% or 95% jumps.
• The same proportional progression applies to every supported track duration.
• Expired/overrunning track sessions use deep purple with white text while showing 00:00 remaining and continuing elapsed time.
• Every generated background selects the higher-contrast black or white timer text.
• Rendering does not allocate a second framebuffer and remains responsive to the stop gesture.
• Existing rest-session colours and timing behavior remain unchanged.
• Host-side tests cover exact anchors, interpolation, clamping, contrast selection, duration scaling, and overrun.
• The feature is exercised on supported physical hardware.

Documentation

• Update the README and User Guide to explain the smooth colour sequence, proportional anchors, overrun state, and automatic high-contrast text.

Relationship

This is an implementation-focused visual improvement beneath roadmap epic #23. It supersedes the abrupt expiry-colour concern in #2. User-configurable warning points and overtime policies remain tracked separately by #29, and alternate accessible themes remain tracked by #34.

[PurpleSentinel]

Implemented, documented, hardware-validated, and merged into v4.1 in #67.