Add full-text search to chat list

[mcintyre94]

Summary

• Adds fullTextContent: String? to SpriteChat — populated on every saveMessages() call by joining all user and assistant text blocks (tool inputs/outputs excluded to keep size reasonable). SwiftData handles the migration automatically since it's optional.
• Wires up .searchable in ChatSwitcherSheet (iPhone modal sheet) and SpriteNavigationPanel (iPad/Mac sidebar), filtering by chat display name or full conversation text.
• No backfill for existing chats — fullTextContent starts populating on next save; name search still works for older chats.

Test plan

• Open chat switcher on iPhone, type a word from a past conversation — matching chats appear, others are hidden
• Clear search — all chats return
• Search by custom chat name
• On iPad/Mac, verify search field appears in the sidebar Chats section and filters correctly
• Confirm open/closed chat sections both filter correctly in the sidebar
• Create a new chat and send a message, then search for a word from that message — it matches

Notes

SpriteNavigationPanel.swift is missing a #Preview (pre-existing gap, out of scope for this PR).

🤖 Generated with Claude Code