feat: conversation persistence, input expansion, overlay UX

- ConversationStore (JSONL + JSON metadata) wired through Backend trait,
  SessionManager, and TUI (/new, /resume, /convos commands)
- Dynamic input box: grows with content, caps at 40% terminal height,
  scrolls internally. Shift+Enter / Ctrl+J for newlines.
- Slash command autocomplete popup (filters live as you type /)
- Conversation picker overlay for /resume (modal, arrow select, Enter switch)
- Splash bloom animation, color_support module, system prompt builder
- Fixed 8 pre-existing test failures (95/95 pass)
- Archived superseded task docs, consolidated planning references

Cargo.toml

@@ -118,7 +118,7 @@ codegen-units = 1
 strip = true
 
 [features]
-default = []
+default = ["figlet-rs"]
 figlet-rs = ["dep:figlet-rs"]
 cowsay = ["dep:cowsay"]
 tauri-desktop = ["dep:tauri", "dep:tauri-plugin-shell"]