fix(ui): align File backstage navigation across five bindings

[yunfeizhu]

Summary

• Align the File backstage navigation rows, icons, active and hover states, and keyboard focus treatment across React, Vue, Angular, Svelte, and Vanilla.
• Make the desktop back control 40px high and clickable across the full sidebar width, with a centered arrow. Keep a 48px control on narrow screens and allow the desktop navigation to scroll when the window is short.
• Share the Tailwind navigation class definitions used by React and Vue, while keeping the other bindings' rendering framework-native.

This branch is based on main; the search-field changes in #350 are outside this diff.

Verification

• e2e/backstage-nav-spacing.spec.ts: 15 passed across the five demos on this branch.
• Type checks passed for all five bindings (Svelte reported 0 errors and 5 warnings in unrelated files).
• Formatting, lint, E2E neutrality, and targeted dependency/Angular builds passed.

Closes #347.

[github-actions]

Cross-binding parity check

This is advisory only and does not block the PR. It looks at changed file paths, so it cannot tell a genuinely framework-specific change from a parity gap. If it is wrong here, just say so in a comment.

Binding	This PR
React	changed
Vue	changed
Angular	changed
Svelte	changed
Vanilla	changed

Some changed bindings have no test changes

No test files changed under: React, Vue, Angular, Svelte, Vanilla.

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See the parity rule for what is expected. Missing: none.

[ChristopherVR]

@ a few conflicts to resolve @yunfeizhu