Add Explorer right-click integration ("Encrypt with ZeeCrypt")

[TheZeekA]

Idea

Add a Windows Explorer context-menu entry ("Encrypt with ZeeCrypt" / "Decrypt with ZeeCrypt") for files and folders, so users don't have to open the app and drag files in manually.

Why now

The MSI installer (dist/windows/installer.wxs) already writes to the registry for the .pcv file association (Software\Classes\ZeeCrypt.pcv\...). A shell context-menu entry is the same kind of registry work — a shell key under Software\Classes\*\shell\ZeeCryptEncrypt (and/or Directory\shell\... for folders) pointing at the exe with the dropped path as an argument.

Open questions to resolve before implementing

• Does ZeeCrypt.exe currently support being launched with a file path as a command-line argument to auto-populate the drop target? (Check flag usage in src/ZeeCrypt.go — the file association's shell\open\command already does "ZeeCrypt.exe" "%1", so there's likely existing support here, but confirm it also handles being launched with no prior UI state expected, and multiple files at once for multi-select.)
• Should this be a single "ZeeCrypt" entry that opens the app with the file(s) pre-loaded (simplest, most consistent with current UX), or separate "Encrypt"/"Decrypt" entries (faster but needs the shell to guess intent from the extension, which won't always be right for encrypt)?
• Per-user (HKCU\Software\Classes) to match the installer's existing no-admin-required, per-user scope
• Icon for the context menu entry (reuse images/lock.ico)

Scope

Registry-only change to dist/windows/installer.wxs; shouldn't require app code changes if %1 argument handling already works as expected above.