feat(ui): consolidate toolbar and add selection-first comments

[ThisIs-Developer]

Summary

This PR modernizes two connected parts of the editor UI:

• consolidates document and header actions into a clearer icon-first toolbar;
• refreshes theme and private-workspace switches, settings surfaces, dropdown motion, and Release Notes action states;
• replaces the previous comments/suggestions experience with a compact, selection-first Review Mode.

Review Mode

• Select rendered text or supported rich content, choose New, and submit feedback without changing Markdown source.
• Keeps a pending text range visibly selected while automatically placing the real keyboard focus and caret in the comment field.
• Creates durable, visible text highlights after submission; highlights remain visible after reload.
• Supports images, diagrams, rendered math, media, and YAML frontmatter tables as element targets. Code blocks remain excluded.
• Treats YAML cell text as text selection, so a drag selection does not accidentally select or outline the entire table.
• Adds compact review threads with author, timestamp, resolve/reopen, edit, delete, filtering, document/panel synchronization, and long-reply collapse/expand controls.
• Adds nested replies with inline edit, delete, persistence, and positioned actions.
• Uses toolbar-style thread borders at rest; open/hover states change the border to blue only, with no background flash.
• Adds blinking start/end selection markers sized to the highlighted line box.
• Closes an untouched composer and clears its pending selection when the user clicks away; typed drafts remain open.

Why

The previous comments interface was visually dense and mixed review metadata with the document workflow. The new flow keeps feedback anchored to rendered content, makes the next action obvious, and preserves a compact, consistent application UI across desktop and mobile layouts.

Data and compatibility

• Review threads stay outside Markdown content and document exports.
• Existing per-document review storage, Live Share review synchronization, workspace backup/restore, and Secret Workspace behavior continue to use the established review collection.
• Generated desktop resources are refreshed alongside the web assets.

Verification

• npm run check:static
• Comments redesign Playwright suite: 7/7 passed
• Review Mode and review design Playwright suites: 5/5 passed
• Nested reply edit/delete/persistence stress repeat: 3/3 passed

Documentation

No separate documentation change was required for these focused UI and interaction updates; the behavior is covered by the PR description and automated regression tests.

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