Fix composer @mention overlay alignment + no-auto-space + code-block backtick hiding

[Aztec03hub]

Comprehensive composer fix for the @mention/overlay bugs Phil reported.

Root cause: the colored .input-overlay was drifting out of alignment with the underlying <textarea>, and the mention-commit path was injecting an unwanted space.

Fixes:

• Doubling-on-select → transparent ::selection on the overlay
• Horizontal drift → hidden textarea scrollbar (scrollbar-gutter)
• Accumulating vertical drift / raised caret / vertically-offset spellcheck squigglies → explicit line-height: 21px on both layers + exact overlay-template whitespace
• NO auto-space after @mention tab-completion (reverted per Phil) — caret lands immediately after the mention; regression test added
• Code backticks/fences hidden once a span/block completes (inline + block); regression test added

vitest 1214 passing; build clean. Needs a desktop redeploy.

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