feat(apple): type-safe SF Symbols via SFSafeSymbols

Replaces every stringly-typed SF Symbol name with a compile-time-checked
SFSymbol case, mirroring the L10n accessor approach. A mistyped or
OS-unavailable symbol is now a build error instead of a silently blank
glyph at runtime.

Adds the SFSafeSymbols SPM package (project.yml) and migrates all call
sites: Image(systemName:)/Label(systemImage:) -> systemSymbol, and the
five symbol-carrying view properties (AppDestination.systemSymbol,
SettingsRow.icon, AboutPoint.symbol, MethodRow.icon, PolicyOption.icon)
flipped from String to SFSymbol end to end.

apple/VniDrop/UI/Feedback/SnackbarHost.swift

@@ -1,4 +1,5 @@
 import SwiftUI
+import SFSafeSymbols
 
 /// Bottom toast host driven by `UiMessageController`, ported from
 /// `ui/feedback/VniDropSnackbarHost.kt`. Tone drives the accent color; errors get
@@ -52,7 +53,7 @@ struct SnackbarHost: View {
 				.buttonStyle(.borderless)
 			}
 			Button(action: dismiss) {
-				Image(systemName: "xmark")
+				Image(systemSymbol: .xmark)
 					.font(.footnote.weight(.semibold))
 					.foregroundStyle(.secondary)
 					.frame(width: 36, height: 36)