chore: prepare standalone public repo

- Remove Mattermost integration (OAuth, channel bridge, file sync watcher,
  frontend import modal). RefBoard now ships as a self-contained app.
- Replace SSO Login screen with email/password form (+ optional register link
  gated by ALLOW_SELF_REGISTRATION).
- Add SEED_ADMIN_EMAIL / SEED_ADMIN_PASSWORD env-var bootstrap so a fresh
  install ships with an admin account on first boot (idempotent).
- ALLOW_SELF_REGISTRATION flag (default false) gates POST /api/auth/register.
  First user can always register (auto-promoted to admin).
- Drop mattermost_id and mm_file_id columns + board_channel_links table
  from the schema; remove related db helpers and exports.
- Add MIT LICENSE, comprehensive README, .env.example, docker-compose.yml
  (bundles MinIO so one command boots a working stack).
- Expand .gitignore for typical Node + Docker dev artefacts.

frontend/src/components/Toolbar.tsx

@@ -30,7 +30,6 @@ interface ToolbarProps {
   onToggleLayers?: () => void;
   showLayers?: boolean;
   onToggleHelp?: () => void;
-  onMmImport?: () => void;
   onExport?: () => void;
   onRefreshPreview?: () => void;
   previewRefreshing?: boolean;
@@ -165,7 +164,6 @@ export default function Toolbar({
   onToggleLayers,
   showLayers,
   onToggleHelp,
-  onMmImport,
   onExport,
   onRefreshPreview,
   previewRefreshing,
@@ -330,16 +328,6 @@ export default function Toolbar({
         </ActionBtn>
       )}
 
-      {/* Mattermost import */}
-      {onMmImport && (
-        <ActionBtn onClick={onMmImport} title="Import from Mattermost">
-          <svg width="14" height="14" viewBox="0 0 14 14" fill="none" stroke="currentColor" strokeWidth="1.3">
-            <path d="M2 10V4a2 2 0 012-2h6a2 2 0 012 2v6a2 2 0 01-2 2H4a2 2 0 01-2-2z" />
-            <path d="M5 7h4M7 5v4" strokeLinecap="round" />
-          </svg>
-        </ActionBtn>
-      )}
-
       {/* Export */}
       {onExport && (
         <ActionBtn onClick={onExport} title="Export as image (Ctrl+Shift+E)">