feat(ui): Modularize stack view and harden config validation

[mhiro2]

Summary

This PR improves stack view maintainability and UX, and hardens config validation behavior.

Changes

• Split stack view into submodules (init / render / tree / keymaps) while keeping compatibility via
  ui/stack_view.lua
• Added configurable stack view panel position: ui.stack_view.position = "left" | "right" | "bottom"
  (default: right)
• Improved stack view rendering behavior:
  • multibyte-safe help width calculation (strdisplaywidth)
  • parser reuse for preview Tree-sitter highlights (same source buffer)
• Extended :PeekstackQuickPeek completion with:
  • lsp.declaration
  • diagnostics.in_buffer
  • marks.buffer, marks.global, marks.all
• Hardened robustness and typing:
  • guard vim.fn.bufadd() return value in popup creation
  • cleaned/improved annotations in stack/persist/init/stack_view
  • updated validate_type() to return and apply fallback defaults