feat(wifi): v1.9.2 - WiFi resilience and credential persistence (#1)

## Problem
Previous versions would clear WiFi credentials on connection failure,
requiring manual reconfiguration via captive portal after each WiFi outage.

## Solution
- Never clear credentials on connection failure
- Infinite retry with exponential backoff (5s → 10s → 20s → ... → 5min max)
- Fallback AP ("TJ56654-Setup") enabled after ~5 min of failed attempts
- Dual STA+AP mode allows configuration while still retrying connection
- AP automatically disabled when WiFi reconnects

## Changes

### WiFi Credential Handling
- Try SDK-stored credentials first (from WiFiManager)
- Fall back to EEPROM config if SDK credentials fail
- Fixes OTA update credential loss issue

### User Experience
- Display shows "No WiFi" with retry countdown instead of cryptic numbers
- "!" indicator in date line when WiFi is disconnected
- Clock continues running with last synced time during outage

### Network Activity (~50 requests/day)
| Service | Interval | Endpoint |
|---------|----------|----------|
| NTP | 1 hour | pool.ntp.org:123 (UDP) |
| Weather | 30 min | api.open-meteo.com (HTTP) |
| mDNS | continuous | 224.0.0.251 (multicast) |

Co-authored-by: Alex Petrochenko <petrochenko@life-pay.ru>

.github/workflows/build.yml

@@ -1,73 +0,0 @@
-name: Build Firmware
-
-on:
-  push:
-    branches: [ main ]
-  pull_request:
-    branches: [ main ]
-  release:
-    types: [ created ]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - name: Checkout code
-      uses: actions/checkout@v3
-
-    - name: Setup Arduino CLI
-      uses: arduino/setup-arduino-cli@v1
-
-    - name: Install ESP8266 platform
-      run: |
-        arduino-cli core update-index
-        arduino-cli core install esp8266:esp8266
-
-    - name: Install libraries
-      run: |
-        arduino-cli lib install "Adafruit GFX Library"
-        arduino-cli lib install "Adafruit SSD1306"
-        arduino-cli lib install "NTPClient"
-        arduino-cli lib install "WiFiManager"
-        arduino-cli lib install "AsyncHTTPRequest_Generic"
-        arduino-cli lib install "ESPAsyncTCP"
-
-    - name: Compile firmware
-      run: |
-        arduino-cli compile --fqbn esp8266:esp8266:generic \
-          --build-property "build.flash_size=1M64" \
-          --build-property "build.flash_mode=dio" \
-          --output-dir build \
-          src/clock_ntp_ota_v1.9.ino
-
-    - name: Check firmware size
-      run: |
-        SIZE=$(stat -c%s "build/clock_ntp_ota_v1.9.ino.bin")
-        echo "Firmware size: $SIZE bytes"
-        MAX_SIZE=470000  # 470KB max for OTA
-        if [ $SIZE -gt $MAX_SIZE ]; then
-          echo "ERROR: Firmware too large ($SIZE > $MAX_SIZE)"
-          exit 1
-        fi
-
-    - name: Upload build artifacts
-      uses: actions/upload-artifact@v3
-      with:
-        name: firmware
-        path: |
-          build/clock_ntp_ota_v1.9.ino.bin
-          build/clock_ntp_ota_v1.9.ino.elf
-          build/clock_ntp_ota_v1.9.ino.map
-        retention-days: 30
-
-    - name: Upload release assets
-      if: github.event_name == 'release'
-      uses: actions/upload-release-asset@v1
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      with:
-        upload_url: ${{ github.event.release.upload_url }}
-        asset_path: build/clock_ntp_ota_v1.9.ino.bin
-        asset_name: esp8266-weather-clock-${{ github.event.release.tag_name }}.bin
-        asset_content_type: application/octet-stream