refactor(website): fold .outlineButton CSS class into the Button outline variant

[theosanderson]

What

The Button component library (introduced in #6604) already defines an outline variant in buttonStyles.ts, but a parallel global .outlineButton CSS class in base.css was still the de-facto standard for outline-style buttons (~10 usages) and was never folded into the library. The two weren't even consistent: the typed outline variant rendered dark/near-black, while .outlineButton rendered primary-coloured.

This PR integrates that style into the component library so there is a single, canonical outline button.

Addresses #6663 (and chips away at the broader button-variety cleanup in #6662).

Changes

• buttonStyles.ts: redefine the outline variant to be primary-coloured (bg-white border-primary-600 text-primary-600 hover:bg-primary-600 hover:text-white), matching the dominant real-world .outlineButton look rather than the lone dark one.
• buttonStyles.ts: standardise the shared button radius on rounded-md (6px) instead of rounded (4px). The migrated outline buttons already used 6px, so this keeps them pixel-identical and nudges every other library button (primary/neutral/ghost) to the same single radius. Circle buttons are unaffected (rounded-full still wins).
• Migrate every .outlineButton call site to variant='outline':
  • DowloadDialogButton.tsx ("Download all entries")
  • DownloadSubmittedDataButton.tsx
  • LinkOutMenu.tsx — a Headless UI MenuButton, so it uses buttonClasses({ variant: 'outline', ... })
  • SeqSetItemActions.tsx (×5) and SeqSetItem.tsx
• Remove .outlineButton from base.css.
• SequenceEntryHistoryMenu.tsx (the version dropdown) previously used the outline variant, so it would have turned primary. It's kept dark instead — to match the black control icons it sits beside in SeqPreviewModal — via one-off utility classes rather than reviving a dark variant in the design system.

The related .outlineButtonDropdownItem class is a menu-item text style (not a button) and is left untouched. The broader "cut down button variety" work (#6662) is separate.

Bonus: no greyed-out flash before hydration

Buttons are disabled until React hydration completes (DisabledUntilHydrated) so Playwright can't interact with them too early. That greyed-out disabled look lived in the shared button base as :disabled pseudo-classes, so it also applied during the brief pre-hydration window — buttons flashed the disabled style on load.

Now the disabled look is applied by intent instead: buttonClasses takes a disabled option (swapping the variant look for the greyed-out look) and the :disabled pseudo-classes are removed from base. The Button component distinguishes "genuinely disabled" (keeps the greyed-out look) from "only waiting for hydration" (keeps the normal enabled appearance and shows a cursor-wait loading cursor). Covered by a new Button.spec.tsx.

Notes

• tsc on the changed files is clean; Button.spec.tsx, the DownloadDialog suite, and SequenceEntryHistoryMenu.spec.tsx pass. The repo's astro check / full tsc currently fail in this environment on pre-existing unrelated missing-dependency issues (flowbite-react plugin, minisearch, exceljs) — present on main too.

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