feat(apple): generate type-safe L10n accessors and migrate all key literals

Replaces every stringly-typed localization key in the Apple app with
compile-time-checked accessors generated from localization/strings.json.
A mistyped key is now a build error instead of a silent fallback to the
raw key at runtime. The runtime path is unchanged: plain keys are
String.LocalizationValue constants resolved with String(localized:) and
Apple's String Catalog still does the lookup; keys with arguments become
typed, named functions applying args through String(format:).

Generator: new renderSwiftAccessors emits apple/VniDrop/Generated/L10n.swift,
wired into generate. Renamed generic arg1/arg2 tokens on four keys to
semantic names (receiver, transferName, deviceId) and updated their context
notes; positional output is unchanged so .xcstrings (bar the 4 comments)
and the Android XML regenerate identical.

Migration: every key-carrying value flipped to String.LocalizationValue
end to end, resolved only at the leaf. Zero key literals and zero
LocalizedStringKey remain in app or test code. macOS build passes; iOS
test run pending.

apple/VniDrop/Resources/Localizable.xcstrings

@@ -1810,7 +1810,7 @@
       }
     },
     "approval_endpoint_id": {
-      "comment": "Approval prompt: shows the requesting device's ID. {arg1} = device/endpoint identifier.",
+      "comment": "Approval prompt: shows the requesting device's ID. {deviceId} = device/endpoint identifier.",
       "extractionState": "manual",
       "localizations": {
         "de": {
@@ -1990,7 +1990,7 @@
       }
     },
     "approval_request_body": {
-      "comment": "Approval prompt body: '{arg1} wants to receive \"{arg2}\".' {arg1} = requester name, {arg2} = transfer name.",
+      "comment": "Approval prompt body: '{receiver} wants to receive \"{transferName}\".' {receiver} = requester name, {transferName} = transfer name.",
       "extractionState": "manual",
       "localizations": {
         "de": {
@@ -9130,7 +9130,7 @@
       }
     },
     "receive_delete_history_description": {
-      "comment": "Receive history: confirmation body for removing one item. {arg1} = transfer name.",
+      "comment": "Receive history: confirmation body for removing one item. {transferName} = transfer name.",
       "extractionState": "manual",
       "localizations": {
         "de": {
@@ -13090,7 +13090,7 @@
       }
     },
     "transfer_delete_description": {
-      "comment": "Transfer details: confirmation body for deleting a transfer. {arg1} = transfer name.",
+      "comment": "Transfer details: confirmation body for deleting a transfer. {transferName} = transfer name.",
       "extractionState": "manual",
       "localizations": {
         "de": {