PMM-15281: Gate SEP's presence in the PMM UI on availability

[peter-o-addo]

Ticket number: PMM-15281

What

Gate SEP navigation and routes on settings.sepEnabled (the UI surface of PMM_ENABLE_SEP). When the integration is off, SEP entries stay out of the sidebar and direct navigation to a SEP route shows an explicit “not enabled” state instead of mounting the auth gate or firing SEP API requests.

SEP remains available to every signed-in user when enabled; write controls stay per-control via canMutate (PMM-15358), not behind isPMMAdmin.

Changes

• Add sepEnabled to ReadonlySettings and default it to false for anonymous users / test stubs
• Push SEP Management nav only when !user.isAnonymous && settings?.sepEnabled
• Gate SepPage on settings: wait while loading/settings is null, then show unavailable when sepEnabled is false
• Cover enabled, disabled, and loading paths in nav and SepPage tests

Notes

• Frontend-only. Consumes sep_enabled from /v1/server/settings and /v1/server/settings/readonly once the backend exposes it; until then the UI treats SEP as disabled (safe default).
• No /health probe — gating is env-var / settings only, matching the updated ticket direction.

If this PR adds, removes or alters one or more API endpoints, please review and update the relevant API documentation as well:

• API Docs updated

If this PR is related to other PRs, contributions, or ongoing work in this or other repositories, please reference them here:

• Depends on a follow-up that surfaces PMM_ENABLE_SEP as sep_enabled on Settings / ReadOnlySettings
• Related nginx / proxy work: PMM-15279 / PMM-15279 Productize the SEP nginx reverse proxy #5759 (PMM_ENABLE_SEP, /sep/ mount)

[it-percona-cla]

All committers have signed the CLA.