Add host metrics preview for TELEMETRY_APP packets

[Copilot]

PORTDUINO (Linux native) hardware sends host metrics in telemetry packets, but these were not decoded in the UI payload preview.

Changes

Backend (packet_routes.py)

• Extract host_metrics from TELEMETRY_APP payloads alongside existing device/environment metrics

Frontend (packet_detail.html)

• Display host metrics with formatting:
  • Uptime: seconds → "12h 17m"
  • Memory/disk: bytes → "17.09 GB" or "189 MB"
  • Load averages: hundredths → "1.25 / 2.00 / 1.50"
  • Optional fields: diskfree2/3, user_string

Tests (test_packet_routes.py)

• Added unit test covering all host metrics fields

Example

Before: Payload showed "Decoded: Yes" with empty "Decoded Data" section

After:

Host Metrics:
  Uptime:         12h 17m
  Free Memory:    189 MB
  Disk Free (/):  17.09 GB
  Load Average:   1.25 / 2.00 / 1.50 (1/5/15 min)
  User Info:      PORTDUINO device on Ubuntu 22.04

Screenshot

Original prompt

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This section details on the original issue you should resolve

<issue_title>Feature Request: Add preview for "Host metrics" in Payload Data</issue_title>
<issue_description>PORTDUINO (Linux native) hardware can send host metrics in a telemetry packet, but Malla doesn't show decoded data for:

Host metrics description from protobuf:

/* Linux host metrics */
typedef struct _meshtastic_HostMetrics {
    /* Host system uptime */
    uint32_t uptime_seconds;
    /* Host system free memory */
    uint64_t freemem_bytes;
    /* Host system disk space free for / */
    uint64_t diskfree1_bytes;
    /* Secondary system disk space free */
    bool has_diskfree2_bytes;
    uint64_t diskfree2_bytes;
    /* Tertiary disk space free */
    bool has_diskfree3_bytes;
    uint64_t diskfree3_bytes;
    /* Host system one minute load in 1/100ths */
    uint16_t load1;
    /* Host system five minute load  in 1/100ths */
    uint16_t load5;
    /* Host system fifteen minute load  in 1/100ths */
    uint16_t load15;
    /* Optional User-provided string for arbitrary host system information
 that doesn't make sense as a dedicated entry. */
    bool has_user_string;
    char user_string[200];
} meshtastic_HostMetrics;

Decoded packet example:

{
      "portnum": 67,
      "payload": "420f08cfd902108080bf5e1880a096b044",
      "want_response": null,
      "dest": null,
      "source": null,
      "request_id": null,
      "reply_id": null,
      "emoji": null,
      "parsed_payload": {
        "host_metrics": {
          "uptime_seconds": 44239,
          "freemem_bytes": "198164480",
          "diskfree1_bytes": "18354638848"
        },
        "type": "protobuf",
        "portnum": "TELEMETRY_APP",
        "message_class": "Telemetry"
      }
    }

I think Copilot could easily implement this.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Feature Request: Add preview for "Host metrics" in Payload Data #56

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