Add Preferences > History pane with CHATHISTORY page-size knob

[buggerman]

Summary

• Add a dedicated History pane in Preferences. Surfaces the otherwise-invisible CHATHISTORY feature (introduced in Surface CHATHISTORY at the top of the channel buffer #21) where users go to configure — and gives the first knob worth exposing: how many older messages to fetch per "Load earlier messages" round-trip.
• Default 50 (matches the previous hardcoded value), with options 50 / 100 / 250 / 500. Clamped to 1–500 at the read site so a manual defaults write can't ask the server for an absurd batch.
• Includes a short explainer that frames what CHATHISTORY does and why the affordance only shows on supporting servers — answers the "is something broken on this server?" question users would otherwise ask.

Changes

• Sources/BryggaCore/PreferencesKeys.swift — new chathistoryPageSize key + chathistoryPageSizeFallback (50) constant.
• Sources/BryggaCore/IRC/IRCSession.swift — new public static func chathistoryPageSize() -> Int that reads + clamps the value. requestMoreHistory(for:) now calls it instead of using a hardcoded 50.
• Sources/Brygga/Views/PreferencesView.swift — new HistoryPane tab with a Picker for the page size and a server-support explainer. Uses clock.arrow.circlepath SF Symbol.
• Tests/IRCSessionTests.swift — three unit tests: fallback when unset, configured value honored, out-of-range clamping.

Test plan

• swift build — passes
• swift test — 121 tests pass (+3 new)
• swiftformat --lint . — clean
• Manual:
  • Open Preferences (Cmd+,) → History tab. Picker defaults to 50.
  • Change to 250. Connect to a server with CHATHISTORY (soju / ergo). Click "Load earlier messages" — observe Brygga sends CHATHISTORY BEFORE <chan> <anchor> 250.
  • Restart the app — picker remembers the chosen value.
  • defaults write <bundle-id> brygga.chathistoryPageSize -int 99999 then click Load — request still uses 500 (clamped).
  • Existing CHATHISTORY behavior on supporting servers is unchanged at the default 50.

Risk / rollback

• Pure additive change. requestMoreHistory semantics are unchanged for users who never visit the new pane (default = previous hardcode).
• Clamping at the read site protects against bad UserDefaults values from any source.
• Revert: this single commit.