Add privacy filter with channel whitelist (GDPR Art. 25)

Introduce an opt-out channel whitelist so the database only persists
messages from channels the user explicitly wants to keep. Default
profile follows GDPR data minimization: own conversations only
(Tells, Party, FC, Linkshells, Cross-World Linkshells, Alliance,
ExtraChat). Public chat (Say/Shout/Yell), Novice Network, NPC
dialogue and system logs are dropped by default.

The filter sits inside MessageStore.UpsertMessage so any current or
future write path is covered uniformly. Configuration provides an
IsAllowedForStorage(ChatType) helper plus a "persist unknown
channels" failsafe (default off) for ChatTypes added by future
patches.

A new Privacy settings tab exposes the whitelist as grouped
checkboxes with three preset buttons (Privacy-First, Clear all,
Select all). Configuration version bumps from 6 to 7; existing
users are migrated to the Privacy-First defaults on first load
and notified once via the Dalamud notification manager.

Also includes a small .env.example and gitignore hygiene for local
development setup.

.gitignore

@@ -3,6 +3,14 @@
 ##
 ## Get latest from https://github.com/github/gitignore/blob/master/VisualStudio.gitignore
 
+# Local development environment (HellionChat fork)
+.env
+.env.bak*
+.envrc
+!.env.example
+.vscode/
+scripts/
+
 # Packaging
 pack/