UI: Add tooltips to Calendar and RangeCalendar navigation buttons

[ciampo]

What?

Add visible tooltips to the previous and next month buttons in the Calendar and RangeCalendar components.

Why?

The icon-only navigation buttons have accessible labels, but sighted pointer and keyboard users cannot see those labels.

How?

Render the navigation controls with IconButton, using the existing localized navigation labels for both the accessible name and tooltip text. Use the compact button size with IconButton's default 24px chevron, and rotate the complete button in RTL layouts while preserving disabled behavior.

Testing Instructions

1. Open the Design System/Components/Calendar/Calendar Default story in Storybook.
2. Hover the previous and next month buttons. Confirm each button shows the matching tooltip and uses the default 24px chevron.
3. Repeat these steps in the Design System/Components/Calendar/RangeCalendar Default story.
4. Switch Storybook to an RTL locale. Confirm both chevrons point in the correct direction.

Testing Instructions for Keyboard

1. In each story, press Tab until the previous month button receives focus. Confirm its tooltip appears.
2. Press Tab again. Confirm the next month button receives focus and its tooltip appears.
3. Press Enter on each button and confirm month navigation still works.

Screenshots or screencast

Use of AI Tools

Codex was used to implement and test this change.

[github-actions]

Size Change: +151 B (0%)

Total Size: 7.9 MB

📦 View Changed

Filename	Size	Change
build/modules/boot/index.min.js	58.1 kB	-5 B (-0.01%)
build/scripts/block-editor/index.min.js	475 kB	+316 B (+0.07%)
build/scripts/block-library/index.min.js	359 kB	+2 B (0%)
build/scripts/edit-post/index.min.js	52.7 kB	+19 B (+0.04%)
build/scripts/edit-site/index.min.js	380 kB	+8 B (0%)
build/scripts/editor/index.min.js	577 kB	-319 B (-0.06%)
build/scripts/format-library/index.min.js	38.9 kB	+10 B (+0.03%)
build/scripts/media-utils/index.min.js	198 kB	+120 B (+0.06%)

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[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: simison <simison@git.wordpress.org>

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[simison]

Copy feels a bit verbose for a tooltip — while it makes perfect sense for a11y text when you don't see the UI. What about "Next month"?

Longer a11y text I'd change to sentence case; "Go to the next month" as that I believe is the style we're moving towards in designs.

[ciampo]

Copy feels a bit verbose for a tooltip — while it makes perfect sense for a11y text when you don't see the UI. What about "Next month"?

Longer a11y text I'd change to sentence case; "Go to the next month" as that I believe is the style we're moving towards in designs.

That label comes from default DayPicker, I'll look into passing a shorter one

[simison]

Sweet!

[ciampo]

Well, it actually was already a first-party label 😅 In any case, I've shortened it as requested.

Thank you for the quick review!

[simison]

I doubt we could rely only on 3rd party library strings anyway, since we need to get the strings into our (and consumers who bundle the UI library) translation pipelines. 🤔