Fix @solidjs/signals resolution and add auto-import completions for Solid 2

[brenelz]

Summary

• Externalize @solidjs/signals in the import map (fixes a runtime crash). The Solid 2 babel preset injects import ... from '@solidjs/signals' into compiled output after the compiler worker has collected externals, so the preview import map never gained an entry for it and the iframe failed with Failed to resolve module specifier "@solidjs/signals". Adding it to EXTERNALIZED gives it a pinned import-map entry, marks it external in the solid-js/@solidjs/web esm.sh URLs (one shared signals runtime instance), and pins its types to the solid-js version so type acquisition no longer relies on jsdelivr's flaky prerelease range resolution.
• Auto-import completions in the TypeScript worker. Typing an unimported symbol (e.g. createProjection) now suggests it and inserts the import statement on accept. Since @codemirror/lsp-client never sends completionItem/resolve and only applies additionalTextEdits present in the initial list, import edits are computed inline for a bounded, prefix-matched set of module-export entries (2+ chars typed, capped at 20), with isIncomplete: true so the client re-queries as the word grows. Entries sourced from @solidjs/signals re-exports and solid-js/types/ internals are skipped to avoid duplicates.
• Prevent the mobile header title from wrapping.

Test plan

• Load a Solid 2 playground, confirm the preview renders without the module-specifier error and that import_map.json gains a pinned @solidjs/signals entry after the first compile.
• Type createProj in an editor tab: createProjection should be suggested and accepting it should add import { createProjection } from "solid-js" (merged into an existing solid-js import when present).
• Confirm Solid 1 playgrounds are unaffected (v1 paths are untouched).

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