test(ui): add automation affordances + rerun discovery against them

The 2026-04-22 discovery run couldn't navigate the sidebar because the
admin UI's nav items are <button (click)="handleNavClick(item)"> with no
routerLink, href, or testid. Force-clicks either timed out on
actionability or fell through to neighboring elements.

This branch fixes that at the source:

Sidebar (df-side-nav.component.html):
* Leaf items are now <a routerLink> instead of <button (click)>. Real
  browser affordances (middle-click / ctrl-click opens in a new tab,
  hover shows URL, copy link works, screen readers announce "link").
  handleNavClick() keeps clearing the error banner but routing now
  happens through [routerLink].
* data-testid on every leaf (`nav-api-connections-event-scripts`) and
  every expansion-panel header (`nav-group-api-connections`).

Common dialog / table buttons:
* data-testid on df-manage-table's `+` and refresh-schema buttons.
* data-testid on df-confirm-dialog's confirm / cancel buttons.

Automation:
* New NavPage page-object (e2e/fixtures/nav.ts) that expands parent
  accordions and routes to any leaf by path.
* Discovery spec (_findings.spec.ts) now uses NavPage; reaches real
  forms on four of five journeys.

First rerun surfaced concrete findings (not fixed here, just reported):
  • api-db: Database create form has zero mat-selects at the position
    where a service-type picker was expected.
  • roles: create form renders only 1 input/select — a role form should
    have many fields. Suggests a lazy-load or licensing gate.
  • admins: /admin-settings/admins path still can't be reached via
    NavPage — the expansion panel opening logic may need another pass.
  • event-scripts: page crashed after picking service=db on the create
    form. Needs a focused repro.

Files prefixed with `_` are exploratory; playwright.config.ts now
ignores them on the default run so CI isn't gated on flaky discovery.

Author: nicdavidson