fix(ui): give icon-only controls a tooltip with their label

[ValentinKolb]

Outcome

Icon-only controls now show their label as the shared @k2b/ui tooltip, in English and German, instead of relying on a late native browser title. This covers the zoom in, zoom out, reset, and fullscreen controls over Mermaid diagrams and other ZoomPanViewport content, the same controls on interactive charts, and every IconButton, IconButtonLink, and icon-only Dropdown.Trigger across the apps.

• Default tooltip: IconButton, IconButtonLink, and iconOnly Dropdown.Trigger show label (or an explicit title) as the tooltip. tooltip still overrides it; tooltip={false} still opts out. A control inside a Tooltip.Anchor leaves the hint to the anchor, so no target shows two tooltips (the existing anchor-wrapped buttons previously showed the anchor tooltip and a native title).
• When it opens: on hover and on visible (keyboard) focus, after the usual 250 ms delay. Touch never opens one. Focus without a ring (after a click or tap, or focus a closing menu or dialog returns silently since fix(ui): return focus without a ring when a pointer opened the overlay #496) opens none, so default tooltips do not pop up after a menu closes.
• Accessibility: the label stays the accessible name. A tooltip that only repeats the aria-label is no longer added as aria-describedby, so screen readers do not read the name twice. Hints that add information, such as Zoom in (+), still describe the control.
• No-JavaScript fallback: server HTML keeps the label as a native title; hydration removes it.
• Library controls: zoom-pan and chart controls, the chart explorer's clear button, split-button and filter-chip menus, chat composer and message actions, markdown editor tools, pane close/add, image rotate, and file-view actions switched from native titles to the shared tooltip.
• Apps: icon-only Button/ButtonLink uses in Core (AI quota, SSH keys), Files shares, Grids (query parameters, custom app builder), and Spaces became IconButton/IconButtonLink. The Cloud preferences menu no longer opts out of its tooltip. The Gateway admin column link got a localized name (EN/DE).
• Guard: the new icon-controls check fails on icon-only Button/ButtonLink (use the IconButton family) and on native icon-only button/a/Tooltip.Trigger without an accessible name.

Field affordances (clear, stepper, remove-chip), tab close, dialog close, and calendar arrows keep their accessible name without a hover hint. They are part of a composite control and are left unchanged here.

Layout

Tooltips are top-layer popovers. A closed tooltip is display: none and sits after its control, so :first-child tap-area rules and row layouts do not change. A real-browser check (Chromium, light mode, EN and DE) showed Zoom in (+) / Vergrößern (+) and Choose list view / Listenansicht wählen on hover, with no change to the control's box and no native title after hydration. A tap on a phone viewport left no tooltip open.

Verification

• packages/ui/test/icon-tooltips.behavior.test.tsx covers: the label tooltip; no title or redundant description after hydration; hover opens it; touch and ring-less focus do not; keyboard focus does; explicit hints describe the control; tooltip={false} and Tooltip.Anchor give one hint; icon-only dropdown triggers and filter chips; and zoom-pan shortcut hints.
• An SSR render test covers the server tooltip and title fallback. Existing render tests are updated for Panes, chat, and Spaces claim.
• scripts/checks/icon-controls.test.ts covers the new rule.
• bun run check (18/18), bun run test, and the @k2b/ui unit, render, and behavior suites pass.

Docs and skill

The UI context is updated: Tooltip (defaults, focus and touch behavior, description rule, SSR fallback), Buttons, Menus, and FilterChip. The cloud-dev skill is unchanged: it already tells authors to use @k2b/ui controls, and the per-component contract lives in the UI context.