[DWAA1660]

What and why

Fixes #156 and #155. Both live in the ink's undo history (inkpad::Pad) and the window's Undo/Redo routing (app.rs). They change the same functions, so they're one PR.

#156: Undo after reopening deleted saved ink. Pad::can_undo returned true whenever the drawing had strokes. With no recorded change, Pad::undo fell back to Drawing::undo(), which popped a stroke loaded from disk. Opening a page and pressing Undo, with no edits, deleted durable ink.

#155: an undone stroke couldn't be redone. Pad kept undo steps but no redo steps. app.redo only called the page-text history, and can_redo ignored ink, so Ctrl+Y said "Nothing to redo".

Changes:

• Pad:
  • undo takes back only recorded changes (undo_steps); the fallback is gone. can_undo is !undo_steps.is_empty(). The page's ink as opened is the baseline.
  • New redo_steps: undo pushes the strokes it replaced, and new redo() / can_redo() put them back (as an undoable step again).
  • settle() clears the redo stack when a new change is recorded, so a new stroke starts a new branch.
• app.rs:
  • undo records what it took back (Undone::Ink, Page or Both) on a small stack. redo pops it and redoes the same target, falling back sensibly if that no longer fits. New ink_redo saves and reports like ink_undo.
  • can_undo / can_redo (Quick Access) and the ribbon's undo/redo flags now include the ink.
  • The record is cleared when another page is opened.

How it was checked

• New inkpad::tests::undo_after_opening_a_page_leaves_its_ink_alone ([BUG] Undo immediately after reopening a page deletes previously saved ink despite no edits in the new session #156): a reopened pad can't undo and keeps its saved stroke, and a stroke drawn after opening can be undone back to exactly the opened state.
• New inkpad::tests::redo_puts_back_what_undo_took ([BUG] Undoing an ink stroke cannot be redone; Ctrl+Y reports Nothing to redo #155): undo twice, redo twice in order, undo after redo, and a new stroke clears redo.
• New UI test undo_and_redo_of_ink_follow_what_was_done_this_session. It draws a real stroke with the pen, then Undo → 0 strokes, Redo → 1 stroke. After going to another page and back, Undo leaves the saved stroke.
• rubbing_a_page_clean_takes_the_file_away_again now adds its stroke the way the pen does, as a recorded change, since a stroke pushed straight into the drawing is no longer undoable.
• cargo test -p notecraft-ui-egui: all pass (304 unit, 202 UI)
• cargo fmt --all -- --check, cargo clippy -p notecraft-ui-egui --all-targets -- -D warnings: clean

🤖 Generated with Claude Code

[DWAA1660]

Follow-up commit: the keyboard's Ctrl+Y / Ctrl+Shift+Z still called step_back(false) directly, so a stroke undone with Ctrl+Z could only be redone with the Redo button, not the key the issue reports. Both keys now go through redo(), like the buttons. Extended undo_and_redo_of_ink_follow_what_was_done_this_session to press Ctrl+Z and Ctrl+Y on a real stroke. cargo test -p notecraft-ui-egui passes (304 unit, 202 UI).