test(ui): nav data-testids + routerLink, discovery spec finds real bugs

[nicdavidson]

Problem

Yesterday's attempt to add a Playwright discovery sweep (PR #487) hit a wall: the admin sidebar isn't automatable. `button.nav-item` elements have no `routerLink`, no `href`, no `data-testid`. Force-clicks either time out on actionability or fall through to nearby elements. Net result: zero bugs surfaced by the discovery run.

What this PR does

Sidebar affordances (df-side-nav.component.html)

• Leaf nav items are now `` instead of `<button (click)>`. Real browser affordances:
  • Middle-click / ctrl-click opens in a new tab (long-standing admin ask)
  • Hover shows URL
  • Copy link address works
  • Screen readers announce "link"
• `data-testid` on every leaf (`nav-api-connections-event-scripts`) and every expansion-panel header (`nav-group-api-connections`).
• `handleNavClick()` kept as a click handler to clear the error banner but routing now happens through `[routerLink]`.

Common UI testids

• `manage-table-create` on the `+` button
• `manage-table-refresh-schema` on the refresh-icon button
• `confirm-dialog-confirm` / `confirm-dialog-cancel` on the generic confirm dialog

Automation infrastructure

• `e2e/fixtures/nav.ts` — `NavPage` object that expands parent accordions and navigates to any leaf by route path.
• `e2e/_findings.spec.ts` — discovery spec that walks five journeys (event-scripts, database API, roles, admins, api-keys) and reports what breaks.
• `playwright.config.ts` ignores `_*.spec.ts` files by default so exploratory runs don't gate CI.

What the discovery spec surfaced on first run

After the nav fix, the discovery reached real forms on 4 of 5 journeys:

Journey	Finding
api-db	Database create form renders zero mat-selects where a service-type picker is expected. Either form control style changed or a control isn't rendering.
roles	Create form has only 1 input/select. A role form should have many fields (name, description, service matrix). Suggests a lazy-load race or licensing gate.
admins	`/admin-settings/admins` still can't be reached via NavPage — the expansion panel opening logic needs another pass. Either the route name differs or the parent group isn't `admin-settings`.
event-scripts	Page crashed after picking `service=db` on the create form. Needs focused repro.
api-keys	Create form loaded cleanly.

Bug fixes not in this PR — these are findings to triage separately.

Test plan

• `npm run build` passes, new dist committed, testids visible in compiled chunks.
• Discovery spec reaches real forms on 4/5 journeys (was 0/5 before).
• Existing functional behavior unchanged (leaf items still route, error banner still cleared on nav).

Out of scope / follow-ups

• Jest regression specs for the 2026-04-22 customer fixes (tracked in PR test: UI testing framework — Jest regression specs + Playwright smoke harness #487 `ui-testing-framework`).
• Resolving the four findings above.
• Playwright nightly workflow wiring (PR test: UI testing framework — Jest regression specs + Playwright smoke harness #487).