Add struct-based NVS storage for ESPHome compatibility

[Copilot]

ESPHome stores preferences using numeric hash keys with binary structs, not individual string-keyed entries. The initial implementation stored WiFi credentials as separate "ssid" and "password" keys, which doesn't match ESPHome's global_preferences->make_preference<T>(hash) API.

Changes

NVS Builder (src/nvs-partition-builder.ts)

• Added BLOB type support for binary data storage
• Implemented packStruct() to pack multiple fields into C struct layout
• Support numeric keys (converted to strings for NVS storage)
• Export packStruct() for reuse

Manifest Schema (src/const.ts)
Extended NVSPartitionConfig with two storage modes:

struct?: {
  key: number;          // ESPHome preference hash (e.g., 88491487)
  fields: Array<{
    name: string;       // Form field to pack
    type: "u8" | "u16" | "u32" | "string";
    maxLength?: number; // Fixed buffer size for strings
  }>;
};

fields?: Array<{       // Legacy mode for individual entries
  name: string;
  key: string;
  type: "u8" | "u16" | "u32" | "string";
}>;

Install Flow (src/install-dialog.ts)

• Pack form values into binary struct when nvsPartition.struct is defined
• Convert boolean form values to numbers for numeric struct fields
• Support both struct and individual field modes

Usage

ESPHome WiFi credentials (correct format):

{
  "customFields": [
    { "name": "wifi_ssid", "label": "WiFi SSID", "type": "text", "required": true },
    { "name": "wifi_password", "label": "WiFi Password", "type": "password", "required": true }
  ],
  "nvsPartition": {
    "offset": 36864,
    "namespace": "esphome",
    "struct": {
      "key": 88491487,
      "fields": [
        { "name": "wifi_ssid", "type": "string", "maxLength": 33 },
        { "name": "wifi_password", "type": "string", "maxLength": 65 }
      ]
    }
  }
}

Matches ESPHome's SavedWifiSettings struct:

struct SavedWifiSettings {
  char ssid[33];
  char password[65];
} PACKED;

The fields mode remains available for non-ESPHome use cases requiring individual NVS entries.

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