Fix Windows taskbar pins after MSI updates and bump to 1.7.3

[danielss-dev]

Windows taskbar pins could retain a Start Menu shortcut icon pointing into the previous MSI product's cache. After an in-app update removed that cache, Strand still launched but the pin showed a blank-page icon.

Use the installed executable's embedded icon for MSI application shortcuts. The custom template matches Tauri CLI 2.11.2 apart from removing the Start Menu shortcut's Icon attribute, preserving the upgrade sequence, Add/Remove Programs icon, and AppUserModelID. Check both the source configuration and compiled MSI shortcuts in the direct and fallback Store release workflows. Bump all desktop version manifests and workspace lockfile entries from 1.7.2 to 1.7.3.

Validation:

• Rust check and frontend TypeScript check passed against current main.
• Local MSI built; both application shortcuts have no cached icon reference and the Start Menu AppUserModelID is retained.
• Regression checks reject the original template and an MSI with ProductIcon restored.
• Release-security, Microsoft Store, MSIX, helper-channel and nine helper-release tests passed.

Existing affected pins require a one-time unpin and re-pin. A full installed-upgrade taskbar rehearsal is tracked in TASKS.md.

Fixes #135