Implement full TollGate display UI

- BOOT screen: centered title + WiFi status line
- READY screen: QR cycling (WiFi/Portal), price, mint domain,
  wallet balance (color-coded), client count
- PAYMENT screen: green banner 'ACCESS GRANTED', paid amount, time
- ERROR screen: red banner 'NO UPSTREAM', guidance, AP reassurance
- Enhanced display_update() API with mint_url, price, wifi_status
- Extract domain helper for clean mint URL display
- Render interval: 2s (reduced SPI load)
- Color palette: cyan, yellow, orange, green, red, dim gray
- WiFi events update display status and trigger ERROR state
- Board C now on /dev/ttyACM3

1 files changed, 136 insertions, 0 deletions

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+# TollGate Display UI Design
+
+## Display Hardware
+- **Panel:** 3.5" IPS, 320x480 portrait (AXS15231B QSPI)
+- **Font:** 8x8 bitmap, scalable (1x=8px, 2x=16px, 3x=24px)
+- **Capabilities:** Text rendering, QR codes, filled rectangles
+- **No touch input** — display is output-only signage
+
+## Color Palette
+
+| Color | RGB565 | Usage |
+|-------|--------|-------|
+| Black | `0x0000` | Background |
+| White | `0xFFFF` | Primary text |
+| Cyan | `0x07FF` | Titles, labels |
+| Yellow | `0xFFE0` | Price, warnings |
+| Green | `0x07E0` | Success, wallet OK |
+| Orange | `0xFD20` | Accent (Bitcoin orange) |
+| Red | `0xF800` | Errors, alerts |
+| Dim gray | `0x8410` | Secondary info |
+| Dark bg | `0x2104` | Card backgrounds |
+
+## Screen States
+
+### 1. BOOT
+Shown during startup until WiFi connects and services start.
+
+```
+┌──────────────────────────┐  0
+│                          │
+│        TollGate          │  y=180, cyan, scale 2
+│      connecting...       │  y=205, yellow, scale 1
+│                          │
+│    WiFi: trying...       │  y=260, dim, scale 1
+│                          │
+└──────────────────────────┘  479
+```
+
+WiFi status line shows: "trying...", "connected!", "failed (retry)"
+
+### 2. READY — QR Cycling (primary screen)
+Cycles every 5 seconds between WiFi QR and Portal QR.
+
+**View A — WiFi QR:**
+```
+┌──────────────────────────┐  0
+│         ┌──────┐         │
+│         │  QR  │         │  QR: WIFI:S:<ssid>;T:nopass;;
+│         │      │         │  Centered in top 2/3 of screen
+│         └──────┘         │
+│                          │  ~y=320
+│   Scan to connect        │  cyan, scale 1
+│   SSID: TollGate-XXXX   │  white, scale 1
+│   21 sats/min            │  orange, scale 1
+│   Wallet: 420 sats       │  green, scale 1
+└──────────────────────────┘  479
+```
+
+**View B — Portal QR:**
+```
+┌──────────────────────────┐  0
+│         ┌──────┐         │
+│         │  QR  │         │  QR: http://10.x.x.x/
+│         │      │         │
+│         └──────┘         │
+│                          │  ~y=320
+│   Portal URL             │  cyan, scale 1
+│   testnut.cashu.space    │  orange, scale 1 (mint domain)
+│   21 sats/min            │  yellow, scale 1
+│   Clients: 3             │  green, scale 1
+└──────────────────────────┘  479
+```
+
+### 3. PAYMENT_RECEIVED
+Shows for 3 seconds after payment, then returns to READY.
+
+```
+┌──────────────────────────┐  0
+│                          │
+│   ████████████████████   │  green filled bar, y=190..230
+│      ACCESS GRANTED      │  white on green, scale 2
+│   ████████████████████   │
+│                          │
+│      Paid: 21 sats       │  white, scale 1
+│      Time: 1 min         │  white, scale 1
+│                          │
+│   Wallet: 441 sats       │  green, scale 1
+└──────────────────────────┘  479
+```
+
+### 4. ERROR
+Shown when upstream WiFi is disconnected.
+
+```
+┌──────────────────────────┐  0
+│   ████████████████████   │  red filled bar, y=190..230
+│     NO UPSTREAM          │  white on red, scale 2
+│   ████████████████████   │
+│                          │
+│   Internet unavailable   │  white, scale 1
+│   Check WiFi config      │  yellow, scale 1
+│                          │
+│   AP still active        │  green, scale 1
+│   SSID: TollGate-XXXX   │  dim, scale 1
+└──────────────────────────┘  479
+```
+
+## Data Flow
+
+### display_update() receives:
+```c
+void display_update(const char *ap_ssid, int active_clients,
+                    uint64_t wallet_balance, const char *portal_url);
+```
+
+### Enhanced to also receive:
+```c
+void display_update(const char *ap_ssid, int active_clients,
+                    uint64_t wallet_balance, const char *portal_url,
+                    const char *mint_url, int price_per_step,
+                    const char *wifi_status);
+```
+
+### display_set_state() triggers:
+- `DISPLAY_BOOT` → at startup
+- `DISPLAY_READY` → when services start (WiFi connected)
+- `DISPLAY_PAYMENT_RECEIVED` → on successful payment (auto-returns to READY)
+- `DISPLAY_ERROR` → when upstream WiFi disconnects
+
+## Implementation Notes
+
+- Render every 2 seconds (reduces SPI bus load vs 1 second)
+- QR codes: auto-size based on string length, centered in top portion
+- Mint URL: show only domain part (truncate at first `/`)
+- Wallet balance: color-coded (green > 100, yellow > 0, red = 0)
+- Client count: "Clients: N" or empty string if 0