Consolidated fork update: battery/power, reliability, history+ETA, mDNS, web UI, diagnostics, CI (+ companion Android app)

[BattloXX]

This PR consolidates all the work done on the BattloXX/free-grilly fork since branching
off. It supersedes my three smaller open PRs (#26, #29, #30), which are all included here —
feel free to close them in favour of this one, or I can rebase this to exclude whatever you
merge first.

master is 23 commits ahead / 1 behind. Everything is backward compatible: new API
fields are additive, and behavioural changes default to the safe/least-surprising option.
A per-release breakdown lives in changelog.md.

Why

The fork chases three goals: (1) make the device stay reachable and not turn itself off,
(2) make long cooks (pulled pork etc.) usable with real history + ETA, and (3) give a
native app a clean, discoverable API to talk to.

Battery & Power

• power_saving setting — a real toggle between "max battery" and "always reachable",
  persisted in NVS, exposed via GET/POST /api/settings and advertised in
  GET /api/info capabilities so an app can gate its UI on it. Defaults to OFF
  (always reachable) after we found the aggressive variant hurt connectivity.
• WiFi modem-sleep (WIFI_PS_MIN_MODEM) actually applied in power-saving mode — the
  single largest battery saving — plus lower task wake-up rates (webserver/MQTT/Opengrill/
  battery/probe loops idle instead of spinning). Full TX power is kept so connections stay
  strong.
• Protective low-battery cutoff — the device has no auto-off timer; it now only shuts
  down to protect the cell at SoC ≤5 % or measured cell voltage ≤3.2 V (needs ~15 s of
  consecutive confirmations, so an I²C glitch can't power it off).

Reliability / "stops turning itself off" fixes

• Reset-reason-aware boot gate — the "hold the button ≥2 s to turn on" gate now applies
  only to deliberate power-ons (cold boot / wake from our own deep sleep). After a fault
  reset (brownout/panic/watchdog) the device resumes instead of silently sleeping, which
  users experienced as an unexplained self-power-off.
• Fixed regular crashes from a shared JsonDocument — every serializer now uses its own
  local document, so the web server, MQTT and Opengrill publishers no longer corrupt a shared
  ArduinoJson pool concurrently. Task stacks enlarged; the coarse-history array moved off a
  small task stack.
• Fixed detail screen crash/reboot on button press — the redraw path now bounds-checks
  the current page against the freshly-read probe count instead of indexing past a shrunk
  list (loose probe → out-of-bounds → panic).
• Debounced power button (GPIO35, input-only, no pull) so electrical noise can't fake a
  long-press shutdown.
• POST /api/settings no longer wipes config — settings are now merged; a partial update
  (e.g. from the app) no longer resets Wi-Fi/AP/MQTT/brightness/timeout fields.
• Display no longer switches off on its own — power-saving no longer overrides an explicit
  "never" (0) timeout; display timeouts are governed purely by the user's settings.

Temperature history & ETA

• Two-tier history buffer for long cooks: a fine tier (last ~30 min @ 10 s) plus a
  coarse tier covering the whole cook at an adaptive interval (starts 60 s, doubles as
  it fills → 3 h → 6 h → 12 h → 24 h …). Memory stays fixed (~5.8 KB total) regardless of
  cook length, so it's battery-neutral.
• ETA / remaining-time estimation (Probe::seconds_to_target(), linear regression over
  recent samples) shown on the LCD and in the API.

Discovery (mDNS)

• Device advertises as free-grilly-<uuid8>.local via _http._tcp and _free-grilly._tcp
  with TXT records (uuid, name, fw) — the app finds devices with zero manual IP entry.

Web UI

• Per-probe Canvas sparkline (no external library) and per-probe ETA.
• Sticky alarm banner + Mute button and browser Notification API for background alerts.
• Energy & Diagnostics on /about: battery %, charging, measured cell voltage, last
  restart reason and last power-off reason — all on-device.
• Fixed hardcoded base_url → relative URLs (works from any network).

On-device LCD

• Detail screen shows ETA (in H:MM) and blinks the probe label when alarming; info screen
  shows the short mDNS hostname.

API additions (all additive / backward compatible)

• GET /api/grill extended: alarm_active, mdns_hostname, per-probe alarm + eta_seconds.
• GET /api/probes/history — fine + coarse history for all 8 probes
  (coarse_interval_seconds, per-probe history_coarse), served from a fixed buffer.
• POST /api/alarm/mute — mute an active alarm.
• GET /api/info — lightweight device identity + capabilities array.
• Status API reports last_off_reason, last_reset_reason, battery_millivolts.
• Android-app compatibility aliases so the app and firmware agree on field names.
• docs/openapi.yaml updated to match.

Build / CI / docs

• tools/generate_web_assets.py regenerates lib/Website/*.h from html_source/
  (gzip for CSS/JS; served Content-Encoding: gzip).
• .github/workflows/release.yml — tag-triggered CI builds OTA + full-flash binaries, runs a
  <2 MB size gate, and attaches both as release assets (also accepts YY.MM.DD-* hotfix tags).
• docs/android_app.md — full API guide (provisioning, NSD discovery, REST reference).

Companion Android app (separate repo — for awareness)

A native Android client was built against this API and is not part of this PR (it lives in
its own repo), but it's the reason several of the API/mDNS/capabilities changes above exist and
may be worth linking from the README:

➜ https://github.com/BattloXX/Free-Grilly-Android (Kotlin · Jetpack Compose · Retrofit ·
Hilt · Room; Android 8.0+)

• Automatic device discovery via mDNS (_free-grilly._tcp) — no IP entry.
• Live temps for up to 8 probes; history graph; background alarm notifications.
• Setup wizard (AP provisioning + Wi-Fi config); power-saving toggle gated on the
  capabilities flag.
• Food library with curated target temps; custom foods + favourites (Room).
• Device status screen with energy (battery %, cell voltage) and diagnostics (last
  restart/power-off reason) — consuming the fields added here.
• In-app OTA firmware update (when the device supports it); phone/tablet adaptive layout;
  bilingual DE/EN; demo mode.

How to review / test

• Per-release detail: changelog.md.
• Firmware build via the existing PlatformIO setup / build_firmware_release.sh, or let the
  new release.yml produce OTA + full binaries on a tag.
• API can be exercised directly (/api/info, /api/grill, /api/probes/history,
  POST /api/alarm/mute) against a device; web UI verifiable at / and /about.