fix(codemirror): keep block controls above the first code line

[huyanxius]

Problem

The code-block language selector and delete button sit over the first code line. Long lines can therefore be hidden even while editing or in read-only mode, as reported in #748.

The toolbar uses the shared absolutely positioned nested-editor toolbar mixin, while the CodeMirror content reserves no space for it.

Changes

• Put only the CodeMirror toolbar in normal document flow and keep its controls right-aligned at the existing top/right edges.
• Let the controls' intrinsic height reserve their own row, avoiding fixed-height assumptions and keeping them available to keyboard and touch users.
• Add a small long-line example with a read-only toggle and browser coverage for desktop/narrow layouts, focus/hover, read-only controls, keyboard language selection/deletion, and touch language selection.

The shared mixin and other nested editors are unchanged. No public API or focus/event-handler changes are involved.

Layout trade-off

Code blocks become taller by one toolbar row. This deliberately keeps both the first line and the controls visible while editing. The earlier issue discussion suggested a hover option; this proposes a CSS-only default layout fix instead, since showing an overlay on hover/focus would still obscure the code during use.

Verification

• Real three-browser red/green run on a standard GitHub-hosted Ubuntu runner: validation run.
  • With the original stylesheet from 122347cff8af5e9e1291b52789d22ed3ba0dc2ba, Chromium, Firefox, and WebKit each failed specifically at the first-line/toolbar geometry assertion. The validation also rejects setup errors or other failure causes.
  • Restoring the candidate stylesheet passed 12/12 layout and interaction cases across the three engines, with no skipped or flaky cases.
  • Keyboard coverage waits for the selected option to receive focus before pressing Enter: Radix defers focus movement after navigation keys with setTimeout.
• npm run lint, npm run typecheck, and npm run build: passed.
• Full unit suite: 122 passed, 1 skipped, 1 todo.
• Browser spec TypeScript check, Prettier, and git diff --check: passed.

The three files here are byte-for-byte identical to those validated on the fork. The fork-only CI harness is excluded from this PR.

Fixes #748