fix(a11y): make team folder edit buttons keyboard-operable

[mvanhorn]

Summary

The team folders admin settings table rendered its row action controls as bare <a> elements with an onClick handler but no href:

• the rename trigger (the mount-point name), and
• the delete control (<a className="icon icon-delete icon-visible">).

Anchors without an href are not part of the tab order and do not respond to Enter/Space activation, so keyboard-only and screen-reader users could not rename or delete a team folder from the settings table.

This PR replaces those non-interactive anchors with native <button type="button"> elements that carry the existing onClick handlers and a discernible aria-label. Buttons are natively focusable and fire on Enter/Space, restoring keyboard operability without a custom keydown handler. Minimal SCSS strips the default button chrome so the controls keep their prior icon/text appearance.

Notable accessibility detail: the rename button's aria-label includes the folder name (Rename "{mountPoint}") so each row remains identifiable to screen-reader and voice-control users rather than every row being announced only as "Rename". The button-chrome reset is scoped to button.action-rename so the sibling <a class="action-rename"> in the groups column keeps its existing link styling.

Why this matters

Closes a WCAG keyboard-operability gap (2.1.1 Keyboard) in the team folders settings UI. The issue was filed by a maintainer and triaged by the Nextcloud team to the actionable 1. to develop state (ref #4466). Scope is intentionally limited to the keyboard-interactivity fix for the row action controls.

Testing

• Keyboard: Tab now reaches each row's rename and delete control, and Enter/Space activates the same handler the click path invokes (rename enters edit mode; delete prompts/removes the folder).
• Screen reader: each control exposes a non-empty accessible name ("Rename """ / "Delete") rather than an empty, non-focusable anchor.
• Visual regression: the icon-only delete button and the inline rename control render with the same size/position/glyph as the prior anchors (no button background, border, or extra padding).
• Mouse: clicking the controls still renames/deletes exactly as before.
• eslint src/settings/App.tsx and stylelint src/settings/App.scss report no new findings on the changed lines (pre-existing warnings/errors unrelated to this change remain).

Fixes #4466

[solracsf]

The rename control now renders as <button class="action-rename">, which no longer matches td.mountpoint a, so font-weight: bold is never applied. The new reset rule (button.action-rename { font: inherit; color: inherit; }) then makes the button inherit font-weight: normal and the ambient text color from td.mountpoint:

td {
    a { color: var(--color-text-lighter); }
    &.mountpoint {
        a { color: var(--color-main-text); font-weight: bold; }
    }
}

Failure scenario: in the admin team-folders table the folder name in each row, previously rendered bold in --color-main-text, now renders at normal weight, a visible regression that directly contradicts the PR's "renders with the same size/position/glyph as the prior anchors" claim. Fix: add font-weight: bold; (and color: var(--color-main-text); if you want to be exact) to the button.action-rename reset, or broaden the td.mountpoint selector to include button.action-rename.