[ZEPPELIN-6358] Add E2E test coverage for notebook components

[dididy]

What is this PR for?

This is the final PR in the series derived from #5101.

Notebook features had zero E2E coverage. This adds 20 spec files (~3500 lines).

Notebook core

• notebook-container — structure, action bar presence, sidebar width constraints, paragraph grid layout, extension area
• action-bar-functionality — run all, code/output toggle, clear output, clone/export/reload, collaboration mode, revision controls, scheduler, settings group
• notebook-keyboard-shortcuts — full ShortcutsMap coverage (Monaco editor; serial because Monaco holds focus state between tests — isolating via beforeEach wasn't viable)
• sidebar-functionality — TOC panel, file tree panel, open/close state transitions
• paragraph-functionality — edit mode, run/cancel, dynamic forms, footer DOM presence

Share features

• folder-rename — hover context menu, rename modal, validation, delete confirmation, folder merge on name collision
• note-rename — inline title editing, enter/blur/escape flows, empty name rejection, special characters
• note-toc — panel open/close, empty state message, toggle button attributes, repeated toggle

Pulled in test failure fixes from #5180

• Cleaned up about-zeppelin-modal and note-create-modal specs and models
• Added missing aria attributes and data-testid selectors to action-bar.component.html
• Bumped flatted 3.3.3 → 3.4.1 (npm audit)

What type of PR is it?

Improvement
Feature
Documentation

Todos

What is the Jira issue?

ZEPPELIN-6358

How should this be tested?

Screenshots (if appropriate)

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[jongyoul]

Merged into master (5e9847b).
Cherry-picked into branch-0.12.