Add dashboard keyboard shortcuts

[rohithreddykota]

Adds dashboard-scoped keyboard shortcuts and a discoverable help dialog.

• Press ? to show or hide the keyboard-shortcuts menu.
• Press /, ,, or . to open the filter, metric, or dimension picker.
• Lists the existing cell inspection, selection, chart navigation, zoom, and explain shortcuts in the same menu.
• Ignores shortcut keys while typing, when modifiers are held, or during repeated keydown events.

Tested with npm test --workspace web-common -- src/features/dashboards/shortcuts/dashboard-shortcuts.spec.ts (9 tests passing).

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[AdityaHegde]

Code wise this looks good. The only thing missing is localisation and text is hardcoded in english.

Feature wise, do we need more keyboard shortcuts? Feels wierd to be able to open the filter dropdown for example and arrow keys don't work.