feat: hide checked items toggle for view mode

[oobagi]

Problem

There's no way to collapse completed checklist items out of sight. When a note has many checked todos, the live/unchecked items get buried. Users want a toggle — primarily in view mode, possibly in edit mode — that hides Checked == true checklist blocks entirely.

Context

• Existing HideChecked config key in internal/config/config.go:18 is misleadingly named — it actually drives sort-to-bottom via m.sortChecked / sortCheckedToBottom(), not hiding. See project memory Sort-checked and the 2026-04-08 work.
• View-mode rendering: internal/editor/editor.go viewMode flag (line 103) and render paths around lines 2540 / 2773 / 2891 / 2909 / 3035.
• Block iteration for rendering happens over m.blocks; hiding likely means skipping blocks with Type == block.Checklist && Checked == true during render and viewport layout.

Approach

1. Rename or disambiguate: either rename the existing config key to SortChecked and introduce a new HideChecked key, or keep HideChecked meaning "hide" and rename the sort-to-bottom flag. Pick whichever minimizes migration pain; document in CHANGELOG.
2. Add a runtime toggle (keybind, e.g., Ctrl+Shift+H, or via footer popup) that flips the in-memory flag for the current session.
3. In the view-mode render pipeline, skip hidden blocks. Viewport height calculation must exclude them too, so scrolling stays correct.
4. Decide edit-mode behavior: either supported symmetrically or scoped to view mode only for v1. Recommend view-mode-only first — editing a list while items vanish is confusing.

Tasks

• Resolve the HideChecked config-name conflict (migration + CHANGELOG note).
• Add rendering skip for checked checklist blocks when the flag is on.
• Fix viewport/scroll math so hidden blocks don't leave gaps.
• Add a keybind + footer popup toggle.
• Decide + implement edit-mode behavior (or explicitly defer).
• Tests for: all-hidden note, mixed, nested checklist children of hidden parents, toggle round-trip.

Acceptance criteria

• Toggling "hide checked" in view mode immediately removes checked items from the rendered output.
• Scrolling, block count indicators, and cursor positioning remain correct.
• Underlying .note file is not mutated by toggling.
• Config persists the preference across sessions.