Add hold-to-open operating modes, graphical G meter, and device settings

[PurpleSentinel]

Summary

Add a safe, persistent operating-mode system opened by pressing and continuously holding the touchscreen for five seconds. The menu must offer the existing timer, a new graphical live G meter, and device settings.

This issue brings together and implements the relevant parts of #31 (brightness), #44 (restore defaults), and #48 (live/peak acceleration) within one coherent on-device mode interface. It belongs under roadmap epic #23.

Entry gesture and safety

• Detect a continuous five-second physical touch, measured from the CST816S touch-count state; do not treat the controller's generic long-press gesture as proof of a five-second hold.
• Reset progress immediately when the finger is released before five seconds.
• Make the threshold a named system configuration value, defaulting to 5 seconds.
• Allow entry from the Timer Ready screen and from G Mode.
• Do not enter configuration during an active track/rest session or while waiting for Launch Mode; this prevents accidental interruption of timing.
• Clear any pending swipe/gesture when the hold completes so the same touch cannot operate the menu.
• Show a concise Ready/G-screen hint explaining how to open the mode menu.

Operating-mode menu

Display a round-screen-safe menu with three choices:

1. Timer Mode — existing track/rest/Launch Mode functionality.
2. G Mode — new graphical live accelerometer view.
3. Settings — brightness and restore-default controls.

Interaction:

• Swipe left/right to move between choices.
• Swipe up to select.
• Swipe down to cancel and return to the previously active operating mode.
• Selecting Timer Mode or G Mode persists the choice in user.json and immediately enters that mode.
• Settings is an action/menu, not a persistent operating mode. Leaving Settings returns to the mode menu; cancelling the menu returns to the previously selected operating mode.
• The saved default operating mode is Timer Mode. Existing user.json files must migrate automatically.

G Mode visual design

G Mode must be graphical rather than a numeric telemetry page.

• Use a circular G-meter illustration drawn in the native framebuffer:
  • central crosshair and concentric reference rings;
  • a filled live marker showing the current filtered acceleration vector;
  • a contrasting hollow/ghost marker showing the maximum vector reached since entry/reset;
  • a short fading trail so motion direction and intensity are visually understandable;
  • a perimeter peak arc or equivalent graphical cue showing maximum magnitude relative to the supported display scale.
• Calibrate a stationary baseline on entry and subtract it so gravity and mounting orientation do not dominate the live display.
• Low-pass filter live X/Y acceleration to reduce jitter.
• Clamp drawing to the circular display and the configured visual scale (the QMI8658 is currently configured for ±8 g).
• Preserve the peak marker until reset. Double-tap resets the peak/trail without leaving G Mode.
• A five-second hold opens the operating-mode menu.
• Target a bounded refresh rate around 10 Hz so touch polling remains responsive and unnecessary LCD transfers are avoided.
• If the QMI8658 is unavailable or fails during sampling, show an actionable message and return safely to the mode menu/Timer Mode; Timer Mode must remain usable.

Settings

Brightness

• Offer clear percentage choices: 25%, 50%, 75%, and 100%.
• Preview brightness immediately while editing.
• Swipe up saves the selection to user.json; swipe down cancels and restores the previous level.
• Apply saved brightness during startup before normal operating screens.

Restore defaults

• Require a confirmation screen before changing data.
• Restore all user-owned settings to built-in defaults: Timer Mode, 100% brightness, track/rest durations, and Launch sensitivity.
• Persist the complete canonical default dictionary atomically.
• Return to Timer Mode after confirmation.
• Cancelling must leave all settings unchanged.

Configuration and compatibility

• Add MODE_MENU_HOLD_SEC (default 5) to system configuration in params.json.
• Add canonical OPERATING_MODE and BRIGHTNESS_PERCENT keys to user.json/built-in defaults.
• Validate supported modes and brightness values, migrate existing files, and preserve safe atomic write/recovery behavior.
• Avoid allocating optional mode/G-meter modules before the RP2040's 115,200-byte LCD framebuffer.

User interface and exit behavior

• Every selection/edit screen must include visible gesture prompts.
• Text and graphics must fit within the 240×240 round display and use high-contrast colours.
• Timer Mode returns to its existing Ready screen.
• G Mode remains active until a five-second hold opens the menu.
• Settings always provides Back/cancel routes and ultimately returns to the selected operating mode.

Acceptance criteria

• A continuous touch enters the mode menu at five seconds; a release at any earlier point does not.
• The menu cannot interrupt a running track/rest/launch session.
• Timer Mode preserves all existing behavior.
• G Mode displays live and maximum acceleration graphically, including a live marker and distinguishable peak marker, without relying on current/max numeric readouts.
• Double-tap clears G Mode peak history.
• Mode and brightness persist across restart.
• Brightness preview/save/cancel behavior works at all supported levels.
• Restore defaults requires confirmation and restores the complete canonical default set.
• Missing IMU operation degrades safely without blocking Timer Mode.
• Existing user/system configuration files migrate safely.
• Host tests cover hold timing/reset, menu navigation/cancel, settings persistence/default reset, G-vector filtering/peak/reset/clamping, display bounds, and IMU failure behavior.
• The full host suite passes and the mode menu/G meter are exercised on the supported Waveshare RP2040-Touch-LCD-1.28.

Documentation

Update the README, User Guide, installation file list, configuration-key documentation, operating-mode gestures, G-meter legend, brightness behavior, default reset warning, and degraded-IMU behavior.

[PurpleSentinel]

Implemented and verified in #60, merged into the v4 default branch for release v4.0.0.