feat(icons): clean tray badge set + Start menu/installer icon

New cmd/icon-gen renders the four state badges (Red/Yellow/Green/Blue) as a
rounded-square badge with a subtle vertical gradient and a crisp white theta,
256px with 4x4 supersampling. The tray embeds these (icons.go, generated) and
builds a proper multi-size Windows ICO (16..256) via exact box filtering --
replacing the old flat 48px circle and nearest-neighbour scaling.

The installer now bundles theta-agent.ico (multi-size, Blue badge) and uses it
for the Start menu 'Theta Agent Tray' shortcut, the setup.exe icon
(SetupIconFile), and the uninstaller display icon. Dead duplicate icon arrays
in the root package (tray_icons.go) removed.

installer/windows/installer.iss

@@ -43,10 +43,12 @@ ArchitecturesAllowed=x64compatible
 ArchitecturesInstallIn64BitMode=x64compatible
 OutputDir={#AgentDir}
 OutputBaseFilename=theta-agent-{#MyAppVersion}-windows-amd64-setup
+SetupIconFile=theta-agent.ico
 Compression=lzma2
 SolidCompression=yes
 WizardStyle=modern
 UninstallDisplayName={#MyAppName}
+UninstallDisplayIcon={app}\theta-agent.ico
 CloseApplications=no
 MinVersion=10.0.17763
 
@@ -69,15 +71,19 @@ Source: "{#AgentDir}\theta-agent-helper-windows-amd64.exe"; DestDir: "{app}"; Fl
 ; driver is signed; no signature phone-home).
 Source: "{#VendorDir}\wireguard-amd64-0.5.3.msi"; DestDir: "{app}\vendor"; Flags: ignoreversion
 
+; Product icon (Start menu shortcut, uninstaller display icon). Generated by
+; cmd/icon-gen from the same badge the tray uses.
+Source: "theta-agent.ico"; DestDir: "{app}"; Flags: ignoreversion
+
 ; OpenCredential credential provider installer (BSD-3 pGina fork) + the VC++
 ; runtime it needs. Both install silently at [Run].
 Source: "{#VendorDir}\OpenCredentialInstaller-1.0.0.0.exe"; DestDir: "{app}\vendor"; Flags: ignoreversion
 Source: "{#VendorDir}\vc_redist.x64.exe"; DestDir: "{app}\vendor"; Flags: ignoreversion
 
 [Icons]
-Name: "{group}\Theta Agent Tray"; Filename: "{app}\tray\theta-agent-tray-windows-amd64.exe"; Comment: "Theta Agent status tray"
+Name: "{group}\Theta Agent Tray"; Filename: "{app}\tray\theta-agent-tray-windows-amd64.exe"; IconFilename: "{app}\theta-agent.ico"; Comment: "Theta Agent status tray"
 Name: "{group}\Open Agent Config"; Filename: "notepad.exe"; Parameters: "{commonappdata}\Theta42\agent.yml"; Comment: "Open the agent configuration file"
-Name: "{group}\Uninstall Theta Agent"; Filename: "{uninstallexe}"
+Name: "{group}\Uninstall Theta Agent"; Filename: "{uninstallexe}"; IconFilename: "{app}\theta-agent.ico"
 
 [Registry]
 ; Start the tray for every interactive logon.