feat(desktop): lsp diagnostics, hover and completion in editor

[gandarfh]

What

The desktop editor now talks to the httui language server: unknown block aliases get a red squiggle with a message, hovering a {{ref}} shows alias info from the server, and {{ completion offers in-scope aliases plus environment variable names (secrets labeled). Manually validated in a real vault.

How

• src-tauri/src/lsp_sidecar.rs — lazy spawn of httui-lsp (PATH or HTTUI_LSP_BIN) with --db pointing at the app database; the webview cannot speak stdio, so the Rust side owns Content-Length framing and bridges messages as lsp_send invokes one way and lsp:message/lsp:exit events the other.
• src/lib/lsp/transport.ts — @codemirror/lsp-client transport over that bridge, queueing messages until the sidecar is up and re-starting it after exit.
• src/lib/lsp/client.ts — one shared client per app process, with server-pushed diagnostics enabled.
• src/lib/codemirror/cm-lsp.ts — per-file extension (LSP plugin with the vault file URI, server completion, hover tooltips), wired in MarkdownEditor.

The local {{ autocomplete and ref hover stay active alongside the server path on purpose: the local hover is still the only source of run-time values, and the server grammar does not parse $prev yet. The local path is removed once the server reaches feature parity.

Tests

• Rust: framing round-trip through a fake server binary under a mock app (spawn, idempotent start, send, event emission), plus pure framing/parsing cases.
• Frontend: transport queueing/flush ordering, single start, event dispatch, restart after exit, URI building, and extension construction. Full suite: 3170 tests green.

New dependency: @codemirror/lsp-client (approved).