refactor(desktop): ref hover and completion served by the lsp

[gandarfh]

Retires the local {{ref}} hover tooltip and completion sources in the markdown editor — the language server now has full parity (types, last-run values, $prev, numeric segments). Kills the stacked double tooltip.

• The editor autocompletion override replaces ALL sources, which had silenced the server source since the first integration — serverCompletionSource is now in the override list (null-safe without the LSP plugin).
• Dead code removed (-444 lines): markdown hover tooltips, per-block ref completion wrappers, the generic ref source.
• Kept local on purpose: DB schema-aware SQL completion, the {{ref}} visual highlight (now directly tested), the click popover, and cm-autocomplete (consumed by the HTTP form-mode inline editors, outside the document the server sees).

Validated manually by the owner: single server tooltip with type + last value, alias/env/field/$prev. completion, DB SQL completion intact.