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.

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to the `theta-agent` daemon will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+- **New tray icon set** (`cmd/icon-gen`, generated `cmd/theta-agent-tray/icons.go`) — the state badges (Red/Yellow/Green/Blue) are now a rounded-square badge with a subtle vertical gradient and a crisp white theta glyph, rendered at 256px with 4x4 supersampling instead of the old flat 48px circle. Windows gets a proper multi-size ICO (16/24/32/48/64/128/256) built with an exact box filter (was nearest-neighbour over three sizes), so the tray/taskbar icon is sharp at every DPI.
+- **Start menu / installer icon** — `installer/windows/theta-agent.ico` (multi-size, Blue badge) is bundled by the installer and used for the Start menu "Theta Agent Tray" shortcut, the setup.exe's own icon (`SetupIconFile`), and the uninstaller's display icon.
+- Removed the dead duplicate icon byte arrays in the root package's `tray_icons.go` (nothing referenced them; the tray binary carries its own copy).
+
 ## [v2.2.0] - 2026-08-10
 
 ### Added