test: port hotkeys checks to release-compatible e2e

[astandrik]

Follow-up from #4389. The selected historical release suite still contains a Ctrl+K-only test that fails in WebKit. This is a compatibility/backport task for that suite, not a claim that the current main UI has a new shortcut regression.

Evidence

Both run 35725732960 and run 35729238809 fail sidebar/sidebar.test.ts:113 — Pressing Ctrl+K in editor page opens hotkeys panel in the safari/WebKit project, with zero retries. The corresponding Chromium case passes.

The runs use backend local-ydb:26.2.1.14 and UI/tests 15.6.0-hotfix.1 at 85e1e1d44e7df6b0f69b4a81e92d2336fb809941, started by npm start.

• Pinned test presses Control+k, waits 1 second, then receives false from the panel visibility check. The retained trace confirms that keypress and absent panel.
• Pinned UI binding selects Cmd+K or Ctrl+K using platform detection. The trace uses the Desktop Safari Mac user agent; runtime navigator.platform was not captured, so the modifier mismatch remains a hypothesis to confirm.
• Current main's helper already tries Ctrl+K then Meta+K and waits for visibility. Reuse existing test work after checking selectors and platform behavior on the historical UI; do not copy the whole current page model without compatibility validation.

Acceptance

• Record the actual platform, registered modifier, focus and visibility behavior in the failing WebKit scenario.
• Port the smallest applicable existing test fix into a revision compatible with UI 15.6.0-hotfix.1; retain genuine keyboard-event coverage and assertions for panel visibility/title.
• Verify Chromium and WebKit with retries disabled, including repeated focused runs and the full release report. Preserve the selected frontend/backend identity.
• If using ci: allow a separate test revision for release e2e #4390's optional tests_sha, record both SHAs. Leave the default release-pinned test selection unchanged.

Screenshots, video, context and trace are retained in each run's release-e2e-report artifact. No fresh product reproduction or fix is claimed here.