Add contextual root README with architecture, workflows, API contracts, and risk callouts

[Copilot]

The repository lacked a cohesive, contextual README that helps contributors quickly understand firmware architecture, operating modes, integration points, and operational risks. This PR introduces a dense root-level README focused on implementation-relevant context rather than boilerplate.

• Scope and system context

  • Added a top-level project narrative covering firmware purpose, major capabilities, and repository contents (firmware, web assets, hardware schematics, UI artifacts).

• Architecture and module map

  • Documented runtime module responsibilities across audio, core, input, web, wifi, ble, and storage.
  • Added an explicit mode model (BLUETOOTH, SMART_ASSISTANT, WIFI_STREAM, HEADPHONE_JACK) and control flow context.

• Hardware and build intelligence

  • Added pin map with I2S peripheral detail (I2S_NUM_0) and key GPIO assignments.
  • Documented PlatformIO environment matrix (mini, regular, max) and feature-flag orientation.

• Operational workflows and API contract

  • Added concise build/flash/SPIFFS workflows.
  • Documented dashboard endpoints and response semantics for /battery and /nowplaying.

• Security and known-state callouts

  • Added prominent warning for hardcoded Wi-Fi credentials and deployment caveats.
  • Captured current dashboard integration bug (script selectors vs. HTML structure) as known state.

# Build one environment
python -m platformio run -e mini

# Upload firmware + SPIFFS assets
python -m platformio run -e mini -t upload
python -m platformio run -e mini -t uploadfs