Icon picker: move catalogue to LocalAppData, grow default to 100 glyphs

[StuartMeeks]

Follow-up to the icon picker (PR #40), addressing two requests: make the catalogue durable across updates, and broaden the default set.

What & why

• Catalogue relocated to %LOCALAPPDATA%\Snipdeck\icon-catalogue.json (renamed from appsettings.json). Previously it shipped beside the executable, where a Velopack update would overwrite user edits. Now it lives alongside the rest of the app config, so edits survive updates.
• Seeded on first run from a default embedded in the app. The embedded default ships fresh with each release and can't be deleted; on first run it's copied to the LocalAppData file, which is what the app reads thereafter (re-read on each open). The embedded default also serves as the fallback when the user copy is missing or malformed — so a bad edit can't break the picker. Delete the file to restore the default.
• Default set grown from 50 to 100 glyphs, all code points grounded against the official Segoe Fluent Icons font list. Includes the requested commerce/business concepts: billing (PaymentCard), customer (ContactInfo), contract (Certificate), invoice (ReportDocument), person (Contact), marketplace (Shop), orders (ShoppingCart), preview, price list (BulletedList), renewal (RepeatAll), report (AreaChart), transfer (Switch).

How

• New IconCatalogueFileName / IconCatalogueFilePath on DefaultPaths, surfaced through IPathProvider (and its implementers + the test fake), wired in Bootstrap.
• GlyphCatalogueProvider now takes the LocalAppData path, seeds-if-missing (write-then-rename), reads the user file, and falls back to the embedded resource. The catalogue JSON is an EmbeddedResource with a pinned LogicalName.
• CHANGELOG and the picker's empty-state / comments updated to match.

Testing

• 226 Core unit tests pass (no behaviour change to the search/selection VM, so existing GlyphPickerViewModel coverage still applies).
• WinUI head compile-verified on the Windows build agent: 0 warnings, 0 errors, embedded resource resolves.

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