Add per-model tray icon color customization

[watatatata810]

Summary

• Tray badge colors are currently fixed per model (Claude/Codex/Antigravity) and can be hard to distinguish depending on the user's taskbar theme/contrast needs.
• Adds a per-model "Icon Colors" submenu in the tray context menu, letting users pick a custom background + text color for each visible model via the native Windows color picker (ChooseColorW), with a "Reset to Default" to restore the original gradient/branding colors.
• Custom colors are opt-in and persisted in settings.json; when not set, behavior is unchanged from the current gradient-based coloring.

Implementation notes

• IconColorOverride { background, text } threads through tray_icon::create_icon/add/update/sync; when present it replaces the usage-based fill/text/outline colors with the flat user-chosen colors (no staged/gradient coloring for overridden icons, per design intent — just a direct override).
• Settings persistence follows the existing SettingsFile/AppState pattern (hex-encoded strings, #[serde(default)] for backward compatibility with older settings.json files).
• Localization: added 7 new Strings fields across all 10 locales. English and Japanese are hand-translated; the other 8 are best-effort/machine-quality translations — native-speaker review welcome before merge.
• Uses the windows crate's Win32_UI_Controls_Dialogs feature (no new external dependency) for the native color picker, with a WM_INITDIALOG hook to relabel each dialog's title bar ("Background Color" / "Text Color") so users can tell the two prompts apart.

Test plan

• cargo build and cargo clippy clean (no new warnings)
• Manually verified on Windows 11: menu appears only for currently-enabled models, color picker flow (background then text) applies immediately to the tray icon, settings persist across restart, "Reset to Default" restores original coloring, cancelling either dialog leaves settings unchanged
• Verified no regression in existing features (update frequency, model visibility toggle, language switching, start with Windows, reset position, show widget)