feat: RBAC, atomic deployments, fleet backups, and licensing (Pro) (#185)

* feat: add RBAC viewer accounts, atomic deployments, and fleet-wide backups (Pro)

Introduces three Pro-tier features:

- RBAC: Multi-user system with admin/viewer roles, user management UI,
  automatic migration from single-admin credentials, viewer restrictions
  across the entire UI (read-only editor, hidden action buttons)

- Atomic Deployments: Pre-deploy file backup to .sencho-backup/, automatic
  rollback on health probe failure, manual rollback button, health probes
  added to stack updates, webhook-triggered deploys use atomic rollback

- Fleet-Wide Backups: Point-in-time snapshots of compose files across all
  nodes (local + remote), stored centrally in SQLite, per-stack restore
  with optional redeploy, graceful handling of offline nodes

* fix(settings): use correct ProGate prop name in UsersSection

* fix(settings): remove unused isPro prop from UsersSection

* fix(auth): fetch user info after login and setup so isAdmin is set correctly

* feat(pricing): revise pricing strategy and enforce variant-based seat limits

Raise Personal Pro from $49/yr to $69/yr with 3 viewer seats (up from 1).
Add $15/mo billing option for Team Pro. Mark lifetime pricing as a
90-day early-adopter offer. Store Lemon Squeezy variant_name on
activation/validation and enforce seat limits server-side per variant.

* feat(licensing): add Lemon Squeezy checkout, webhook, and billing portal integration

Server-side checkout URL generation (POST /api/checkout) with admin email
pre-fill and instance_id custom data. HMAC-SHA256 verified webhook endpoint
(POST /api/webhooks/lemonsqueezy) handling order, subscription, and payment
lifecycle events for automatic license activation. Customer billing portal
link stored from webhook events and exposed via GET /api/billing/portal.
In-app checkout buttons in Settings with manual license key fallback.

* fix(licensing): exempt Lemon Squeezy webhook from auth middleware

The catch-all auth middleware on /api/* was blocking the public webhook
endpoint. Added /webhooks/lemonsqueezy to the exemption list alongside
/auth/* and /webhooks/:id/trigger.

* feat(pricing): update pricing to final live rates

Personal Pro: $7.99/month, $69.99/year, $249 lifetime.
Team Pro: $49.99/month, $499.99/year, $1,499 lifetime.
Added personal_monthly checkout variant across backend, frontend, and website.

* refactor(licensing): remove server-side checkout/webhook for self-hosted model

Sencho is self-hosted — each user runs their own instance, so there is
no central server to receive webhooks or hold the store API key. Replaced
in-app checkout buttons with a "View Pricing" redirect to sencho.io and
kept manual license key activation as the primary flow.

- Delete LemonSqueezyService (checkout, webhook, HMAC verification)
- Remove POST /api/checkout, GET /api/billing/portal, POST /api/webhooks/lemonsqueezy
- Remove raw body parser and auth exemption for webhook route
- Remove all LEMONSQUEEZY_* env vars from .env.example
- Replace checkout buttons in SettingsModal with single "View Pricing" button
- Simplify LicenseContext checkout to open sencho.io pricing page
- Update licensing docs to reflect website-based purchase flow

* chore: normalize em-dashes to hyphens across codebase (linter)

* chore: remove accidentally tracked directories from index

frontend/src/index.css

@@ -28,7 +28,7 @@
   --input: oklch(0.9400 0 0);
   --ring: oklch(0.50 0.14 200);
 
-  /* Brand — technical cyan, the signature accent */
+  /* Brand - technical cyan, the signature accent */
   --brand: oklch(0.50 0.14 200);
   --brand-foreground: oklch(1 0 0);
   --brand-muted: oklch(0.50 0.14 200 / 0.12);
@@ -110,7 +110,7 @@
   --input: oklch(0.3200 0 0);
   --ring: oklch(0.72 0.14 200);
 
-  /* Brand — bright technical cyan on dark backgrounds */
+  /* Brand - bright technical cyan on dark backgrounds */
   --brand: oklch(0.72 0.14 200);
   --brand-foreground: oklch(0.05 0 0);
   --brand-muted: oklch(0.72 0.14 200 / 0.12);
@@ -140,7 +140,7 @@
   /* Shape */
   --radius: 0.5rem;
 
-  /* Shadows — deeper on dark backgrounds */
+  /* Shadows - deeper on dark backgrounds */
   --shadow-x: 0px;
   --shadow-y: 1px;
   --shadow-blur: 2px;
@@ -331,7 +331,7 @@ body {
 .animate-delay-300 { animation-delay: 300ms; }
 
 /* ─────────────────────────────────────────────────────────────
-   PREFERS REDUCED MOTION — zero-cost accessibility
+   PREFERS REDUCED MOTION - zero-cost accessibility
 ───────────────────────────────────────────────────────────── */
 @media (prefers-reduced-motion: reduce) {
   *,