fix(installer): silent install kept empty server_url; tray now starts; self-update uses GitHub releases

Fresh-install bug report fixes:

- Silent installs wrote server_url: '' -- CurPageChanged fires even when the
  wizard is walked programmatically in silent mode, so it read the empty edit
  boxes and clobbered the /SERVER_URL /JOIN_KEY command-line params. Guard the
  read with WizardSilent() so silent installs keep the params and interactive
  installs keep the wizard values. (This is also why the service exited on
  first connect and the Directory showed the agent as 'v2.0.0': the agent never
  connected, so the server fell back to its placeholder version.)
- The tray post-install launch had skipifsilent, so a silent install (the
  common path from the Directory's Windows command) never started the tray.
  Removed it -- the tray starts in silent installs too.
- theta-agent update (cli.go) downloaded from the SSO /resources path, which no
  longer serves binaries (they are GitHub release artifacts now) -- 404.
  Pointed it at releases/latest/download via releaseAssetURL.

installer/windows/installer.iss

@@ -91,8 +91,10 @@ Filename: "msiexec.exe"; Parameters: "/i ""{app}\vendor\wireguard-amd64-0.5.3.ms
 Filename: "taskkill.exe"; Parameters: "/f /im wireguard.exe"; Flags: runhidden
 ; Register the agent as a SYSTEM auto-start service.
 Filename: "{app}\{#MyAppExeName}"; Parameters: "install-service"; StatusMsg: "Registering theta-agent service..."; Flags: runhidden waituntilterminated
-; Show the tray right away instead of waiting for the next logon.
-Filename: "{app}\tray\theta-agent-tray-windows-amd64.exe"; Description: "Start Theta Agent tray"; StatusMsg: "Starting Theta Agent tray..."; Flags: nowait postinstall skipifsilent
+; Show the tray right away, in silent and interactive installs alike (a silent
+; install is the common path from the Directory's install command, and the tray
+; should appear immediately there too, not only at the next logon).
+Filename: "{app}\tray\theta-agent-tray-windows-amd64.exe"; Description: "Start Theta Agent tray"; StatusMsg: "Starting Theta Agent tray..."; Flags: nowait postinstall
 
 [Code]
 var
@@ -206,11 +208,15 @@ begin
   CreateAgentConfigPage();
 end;
 
-// Pull the values the operator typed into the wizard so WriteAgentConfig can use
-// them; silent installs keep the command-line params.
+// Pull the values the operator typed into the wizard so WriteAgentConfig can
+// use them. In silent mode the wizard is walked programmatically and
+// CurPageChanged fires too -- reading the (empty) edit boxes there would wipe
+// the /SERVER_URL /JOIN_KEY command-line params and leave agent.yml with an
+// empty server_url. Only take the edit values when the wizard is actually
+// being shown interactively.
 procedure CurPageChanged(CurPageID: Integer);
 begin
-  if CurPageID = AgentConfigPage.ID then begin
+  if (CurPageID = AgentConfigPage.ID) and (not WizardSilent()) then begin
     ServerURL := Trim(ServerURLEdit.Text);
     JoinKey := Trim(JoinKeyEdit.Text);
   end;