feat(tui): add Alt+S to stash input text, show stash hint when input is present

[alecuba16]

Summary

Adds Alt+S to stash input text. Stash hint (alt+s stash / alt+s restore stashed input) now shows when input text is present, not just when empty alongside ? shortcuts.

Changes

• render.rs: Rewrote right_hint logic in render_input():
  • Empty input + has stash → alt+s restore stashed input (magenta)
  • Empty input + no stash → ? shortcuts · alt+s stash (dim)
  • Non-empty input + has stash → alt+s restore stashed input (magenta) — NEW
  • Non-empty input + no stash → alt+s stash (dim) — NEW
• Removed old paste ref hint that incorrectly occupied the non-empty input path
• prompt_input.rs: stashed_text: Option<String> field, stash() / restore_stash() methods

Testing

• cargo check --workspace clean
• cargo fmt --all clean
• 119 render tests pass

[alecuba16]

Closing in favor of consolidated group branches. The changes from this PR have been merged into a functional group branch.