feat: toggle to hide checked-off checklist items

[oobagi]

Problem

When a checklist grows long, completed items add clutter. There's no way to hide checked-off items to focus on what's left.

Context

Checklist blocks are defined in internal/block/block.go:85-89 with a Checked bool field. Rendering happens in internal/editor/render.go — active blocks at line 234, inactive at line 595, and view mode at line 738. Toggle between checked/unchecked already works via Ctrl+X (edit) and mouse click (view).

Approach

Add a 3-way HideChecked setting:

• off — show all items (current behavior, default)

• view-only — hide checked items in view mode only, still visible in editor

• on — hide checked items in both editor and view mode

• Store the setting in config (global default) and/or per-note metadata

• In render.go, skip checked Checklist blocks based on the mode + setting

• Add a keybind to cycle through states

• When items are hidden, show a subtle "[N hidden]" indicator so the user knows items exist

Files to touch:

• internal/config/ — new setting
• internal/editor/render.go — filter logic in all three render paths
• internal/editor/editor.go — keybind to cycle the toggle

Tasks

• Add HideChecked config option with values (off, view-only, on)
• Thread the setting through the editor model
• Filter checked blocks out of rendering based on current mode + setting
• Add "[N checked hidden]" indicator when items are filtered
• Add keybind to cycle the toggle (and show current state briefly)
• Persist toggle state per-note or globally

Acceptance criteria

• Checked items disappear from view mode when set to view-only or on
• Checked items disappear from edit mode when set to on
• Hidden count indicator visible when items are filtered
• Cycling the toggle updates the view immediately
• Default is off (no behavior change for existing users)

[oobagi]

Implemented as sort-checked-to-bottom with bundle-aware sorting in PR #201. Ctrl+H toggles, also available in the new settings screen (PR #202).