[RTE] Unify how the TipTap editor determines RTL #6114

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Overview

The TipTap editor resolves the page direction four different ways, so an RTL fix applied to one control does not carry to the others. Reduce them to a single source.

Complexity: Low
Target branch: unstable

Context

• TipTapEditor/composables/useRovingTabIndex.js reads window.isRTL.
• TipTapEditor/components/toolbar/ToolbarButton.vue reads document.dir or <html dir>.
• TipTapEditor/composables/useLinkHandling.js reads document.dir.
• TipTapEditor/components/image/ImageNodeView.vue walks up from the editor DOM to the nearest [dir="rtl"].
• Studio sets window.isRTL in templates/base.html and re-exposes it as Vue.prototype.$isRTL in shared/i18n/setup.js.

The Change

• Every JavaScript RTL check under shared/views/TipTapEditor/ should resolve from one source.
• ImageNodeView's ancestor walk is the only one that can report a direction differing from the page's; whether that difference is worth keeping is part of this task.

Out of Scope

• RTL reads elsewhere in Studio.
• [dir='rtl'] CSS selectors, which RTLCSS handles.

Acceptance Criteria

• Every JavaScript RTL check under shared/views/TipTapEditor/ reads from one source
• In RTL, toolbar icons still flip, the link popover still anchors on the correct side, image resize handles still track the drag direction, and toolbar arrow keys still move in reverse
• Jest tests that exercise RTL set the direction through that single source

References

• Review comment that raised this: Make the rich text editor toolbars a single tab stop with arrow-key navigation #6108 (comment)

AI usage

Claude Code drafted this issue from a review comment on #6108. It grepped shared/views/TipTapEditor/ for the direction checks and listed the four call sites it found; the acceptance criteria were reviewed against those call sites before filing.