feat(auth): add canonical accountUrl helper

[coopbri]

What

Adds accountUrl(accountBaseUrl) to @omnidotdev/providers/react, the canonical "manage account" URL helper. It trims trailing slashes and returns the account base URL; the caller passes a full base URL.

Why

Apps today inline ${AUTH_BASE_URL}/dashboard or a CONSOLE_URL for "manage account", inconsistently (herald-app alone uses both). This is step one of consolidating all account management onto a single host (future account.omni.dev). With one canonical helper plus a per-app VITE_ACCOUNT_URL env, the account host can move with a one-line env change per app instead of edits across the fleet. accountUrl sits beside the existing gatekeeperDashboardUrl, whose doc now steers account links to accountUrl (org-management deep links keep using gatekeeperDashboardUrl for now).

Design

• Trivial by design: only trims trailing slashes, no path params, no env reading (providers never reads env, callers pass the base URL in). Distinct from gatekeeperDashboardUrl, which appends /dashboard.
• Interim value is https://identity.omni.dev/dashboard (passes through unchanged) and flips to https://account.omni.dev later.

Tests

• New colocated src/react/ManageTeamLink.test.ts: base URL unchanged, trailing-slash trim, interim identity-dashboard value preserved.
• Full suite 325 pass; typecheck, Biome, and knip clean. build/ is regenerated by the pre-commit hook; the small build/index.js / build/flags diffs are the hook syncing a previously-stale committed build, not behavioral changes.

Rollout

After merge, repin @omnidotdev/providers to the merged SHA in apps as they adopt accountUrl (starting with herald-app), with no per-app change beyond the link repoint plus VITE_ACCOUNT_URL.