[Feature]: Show list of installed apps and let MSIXplain them

[jvintzel]

Problem

Have a feature to show the installed apps on the system via the GUI and explain what they do.

Proposed Solution

Have a feature to show the installed apps on the system via the GUI and explain what they do.

Alternatives Considered

No response

Component

CLI

[aclinick]

Closing — shipped in #14 (auto-merging into main).

What was built

Installed Apps pane (the core ask):

• Click the new Apps entry in the sidebar to open an Outlook-style pane listing every MSIX-packaged app currently installed on the machine, with name, publisher, version, architecture, and (lazy-loaded) tile icon.
• A start-menu-style filter narrows the list to user-facing apps (vs. resource/framework packages).
• Pick any app and MSIXplainer explains it the same way it explains a .msix you've opened from disk — same Sections pane, same rules engine, same export options.

Compare against an installed app:

• The Old/New pickers in Compare accept a path or a selected installed app, so IT pros can answer "what would the update of this installed app cost on the wire?" without first hunting down the original .msix.

While we were in there:

• New Compare sidebar: Diff / Bandwidth Planner / Duplicates with badge counts.
• Swap button between Old and New package pickers.
• New Settings page (gear in NavView footer): Light / Dark / Use system theme + About card.
• CLI: msixplainer --version / -v; version line in the banner; assembly version stamped from the package manifest so CLI, WinUI About, and the .msixbundle all report the same number.
• Several real-world bundle parsing fixes uncovered while compare-testing Microsoft.DesktopAppInstaller (stub packages, multiple same-arch inner packages).

Tests: 149 / 149 passing.