Encrypt Discord token at rest in settings file (machine-bound) (#1491)

* Initial plan

* Add token encryption when saving/loading settings

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Apply suggestion from @Tyrrrz

* Apply suggestion from @Tyrrrz

* Bind token encryption key to machine identity

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Switch to AES-GCM, hex encoding, and GetBytes/Fill improvements

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Address all review feedback: salt injection, code style, localization formatting

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

* Address latest review: ThisAssembly.Project, EnvironmentExtensions, inline Lazy, renames, localization wording

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* Address latest review: layout comment, cipherSource, else block, MachineName fallback, csproj ordering

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* Apply suggestion from @Tyrrrz

* Rename GetMachineId→TryGetMachineId, refactor Write to use single array with FillBytes

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* Rename cipherSource→cipher in Read(), tokenBytes→tokenData in Write(), update layout comments

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* Add cipherSource variable in Write(), update layout comment with size annotation

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* Fix CSharpier formatting: inline multiline string assignments and reformat exception filter

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* Quote EncryptionSalt argument to handle single quotes in secret value

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* Revert double-quote fix on EncryptionSalt argument

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* Apply suggestion from @Copilot

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

DiscordChatExporter.Gui/Localization/LocalizationManager.English.cs

@@ -52,8 +52,10 @@ public partial class LocalizationManager
             [nameof(AutoUpdateLabel)] = "Auto-update",
             [nameof(AutoUpdateTooltip)] = "Perform automatic updates on every launch",
             [nameof(PersistTokenLabel)] = "Persist token",
-            [nameof(PersistTokenTooltip)] =
-                "Save the last used token to a file so that it can be persisted between sessions",
+            [nameof(PersistTokenTooltip)] = """
+                Save the last used token to a file so that it can be persisted between sessions.
+                **Warning**: although the token is stored with encryption, it may still be recovered by an attacker who has access to your system.
+                """,
             [nameof(RateLimitPreferenceLabel)] = "Rate limit preference",
             [nameof(RateLimitPreferenceTooltip)] =
                 "Whether to respect advisory rate limits. If disabled, only hard rate limits (i.e. 429 responses) will be respected.",