Improve DX Cluster custom telnet flow, spot-time handling, and DX panel relative time

[echo-gravitas]

PR Summary

This PR improves DX Cluster behavior end-to-end, especially for source=custom, and adds relative spot time display in the DX panel.

1) Fixed custom endpoint hangs

• Fixed a Promise resolution bug in tryDXSpiderNode() that could leave /api/dxcluster/paths?source=custom... hanging indefinitely on connect/timeout/error paths.
• Ensured all code paths resolve cleanly.

File: server.js

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2) Added .env support for custom telnet defaults

• source=custom now supports backend defaults from environment/config:
  • DX_CLUSTER_HOST
  • DX_CLUSTER_PORT
  • DX_CLUSTER_CALLSIGN
• Frontend query values still override defaults when provided.
• Callsign resolution now follows:
  1. request callsign
  2. DX_CLUSTER_CALLSIGN
  3. CALLSIGN-56
  4. GUEST

Files: server.js, .env.example

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3) Improved DX spot parsing (Telnet lines or HTML table format)

• Added parsing support for DXSpider sh/dx table output (not only DX de ... lines).
• This significantly increases parsed spots for cluster nodes that return table-style data.

File: server.js

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4) Kept spots without coordinates in list data

• Spots are no longer dropped when station coordinates cannot be resolved.
• Map rendering still filters to valid coordinate pairs, but the DX list can now show non-mappable spots.

File: server.js

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5) Reworked source=custom to persistent telnet session

• Replaced reconnect-per-poll behavior with persistent custom sessions:
  • per host/port/callsign session key
  • keepalive + reconnect handling
  • in-memory rolling spot buffer
• sh/dx 25 is sent once per session lifecycle (initial snapshot), not on every frontend poll.
• set/dx is sent on connect to keep live stream updates.
• Disabled HamQTH fallback when source=custom is explicitly selected.

File: server.js

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6) Added explicit command logging

• Added console log lines when sh/dx 25 is sent, including host/port/callsign.
• Uses logInfo, so visible with LOG_LEVEL=info or debug.

File: server.js

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7) Sorting now uses spot time (HHMMz) as primary source

• Added explicit conversion of spot-provided HH:MMz to UTC timestamp (with day rollover handling).
• Path entries now use this time as authoritative for ordering (fallback only if unavailable).

File: server.js

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8) DX panel: relative time display

• Added relative time display in DX Cluster panel based on spot time.

File: src/components/DXClusterPanel.jsx

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9) i18n updates for relative DX time

• Added dxClusterPanel.relativeTime key to all language files.
• German uses German phrasing; all other languages use English fallback text as requested.

Files:

• src/lang/de.json
• src/lang/en.json
• src/lang/es.json
• src/lang/fr.json
• src/lang/it.json
• src/lang/ja.json
• src/lang/ko.json
• src/lang/nl.json
• src/lang/pt.json
• src/lang/sl.json

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10) Tooling and consistency updates

• Added .editorconfig to enforce consistent base formatting across editors/IDEs.
• Added .prettierrc to define a shared formatting standard for the codebase.
• Removed package-lock.json from .gitignore so lockfile changes are tracked in Git.

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11) Dependency reproducibility policy

• Lockfile is now part of version control to ensure deterministic dependency resolution across all contributors/CI.
• Recommended install workflow is npm ci (instead of npm install) to guarantee everyone uses the exact locked dependency tree.