Fix External MCP server bugs; add npx/Node.js detection and install help

Root-caused two real issues Rune hit with Obsidian/Homepage external MCP
servers:

1. Toggling a server's enable switch silently wiped transportKind/env/
   url/bearerToken/headers back to stdio defaults (only id/name/command/
   args/isEnabled/timeout/createdAt were preserved) — almost certainly
   how Obsidian's config got corrupted into an empty-command stdio entry
   despite never being edited directly. Fixed via
   ExternalMCPServer.withEnabledToggled(), which flips only isEnabled.

2. npx (installed via Homebrew) was invisible to Confab because GUI apps
   only inherit launchd's minimal PATH, not the Terminal PATH. Tried
   spawning the user's login shell to ask for its real PATH — this
   caused two real hangs in one session (first an -ilc pipe deadlock,
   then a waitUntilExit()/CFRunLoop reentrancy issue even after fixing
   that) and was abandoned entirely in favor of LoginShellEnvironment:
   deterministic, subprocess-free directory probing (Homebrew, MacPorts,
   Volta, nvm's alias file) that can't hang by construction.

Also added:
- Edit capability for existing External MCP servers (previously only
  Add/Toggle/Delete) — the second thing Rune explicitly asked for, and
  the way to fix a corrupted entry like Obsidian's without deleting it.
- MCPClientError.commandNotFound: a stdio server's command is checked
  against PATH up front in StdioMCPTransport.prepare() and fails
  immediately with a clear reason instead of cycling through 3 rounds of
  crash/restart backoff (5s/15s/30s) for a permanently-missing binary.
- A "Get Node.js" button appears when this happens, opening a sheet with
  a copyable `brew install node`, a one-click install (via
  NodeInstallHelper, using the terminationHandler/readabilityHandler
  pattern already proven safe elsewhere in this file — deliberately not
  waitUntilExit()), or a nodejs.org link if Homebrew isn't present.
- ExternalMCPManager.retryClient(id:) to manually retry after fixing the
  underlying cause.
- Help book: new "Servers That Use npx" section, updated Server Status
  section, updated Settings blurb.

37 new/changed tests covering the toggle fix, PATH probing, the
commandNotFound fast-fail path, and missing-command detection — full
suite (374 tests) passes clean.

oAI/Services/ExternalMCPModels.swift

@@ -123,6 +123,16 @@ nonisolated struct ExternalMCPServer: Codable, Identifiable, Sendable {
     ]
 
     var isSlugReserved: Bool { Self.reservedSlugs.contains(slug) }
+
+    /// Returns a copy with only `isEnabled` flipped. Exists so toggling a server in Settings can't
+    /// silently drop fields — see the fixed bug in `SettingsService.toggleExternalMCPServer`, which
+    /// used to rebuild the struct from a subset of fields and lose `transportKind`/`env`/`url`/
+    /// `bearerToken`/`headers` on every toggle.
+    func withEnabledToggled() -> ExternalMCPServer {
+        var copy = self
+        copy.isEnabled.toggle()
+        return copy
+    }
 }
 
 // MARK: - Client State
@@ -154,6 +164,12 @@ enum MCPClientError: LocalizedError {
     case handshakeFailed(String)
     case writeFailed
     case invalidConfiguration(String)
+    /// The stdio server's `command` couldn't be located anywhere on PATH (inherited + the common
+    /// install directories `LoginShellEnvironment` checks). Deliberately distinct from
+    /// `.processLaunchFailed`: this is a permanent, static condition — no amount of
+    /// restart-with-backoff will ever fix a binary that isn't there — so callers route it straight
+    /// to a clear `.error` state instead of the crash/retry loop. See `ExternalMCPClient.start()`.
+    case commandNotFound(String)
 
     var errorDescription: String? {
         switch self {
@@ -164,6 +180,7 @@ enum MCPClientError: LocalizedError {
         case .handshakeFailed(let s):     return "MCP handshake failed: \(s)"
         case .writeFailed:                return "Failed to write to MCP server stdin"
         case .invalidConfiguration(let s): return "Invalid MCP server configuration: \(s)"
+        case .commandNotFound(let s):      return "Command not found: \(s)"
         }
     }
 }