feat: add document layers

[awerty-noob]

Summary

Add document layers (similar to image editors like Photoshop/GIMP) to Rnote. Closes #1253.

Each document now has a list of layers with name, visible, locked states. Strokes belong to a layer; only the active layer is drawn/edited on by default.

Changes

Data model (crates/rnote-engine/src/store/layer_comp.rs)

• New types: DocumentLayer, DocumentLayerId, LayerComponent.
• StrokeStore gains layers, layer_components, active_layer_id, layer_counter.
• Old .rnote files without layer data are backward-compatible — all strokes default to Layer 1.

Rendering order

Sorting in chrono_comp.rs now respects document layer order → existing StrokeLayer → chrono timestamp.

Per-layer filtering

• Render: only visible layers are rendered (stroke_keys_as_rendered, regenerate_rendering_in_viewport_threaded).
• Selection: only active, editable (visible & unlocked) layer strokes are selectable.
• Eraser: only editable active layer strokes are erased/split.
• Thumbnails: hidden layers are excluded.
• Export: uses the render path, so hidden layers are excluded.

Engine API

Public methods added on Engine:

• layers(), active_layer_id()
• set_active_layer(), add_layer(), delete_layer(), rename_layer()
• set_layer_visible(), set_layer_locked()
• move_layer_up(), move_layer_down()

All layer mutations enter undo/redo history.

Xournal++ (.xopp) import/export preservation

.xopp files now preserve document layer structure on import and export:

• Import: xopp layers are mapped to rnote DocumentLayers by name (merge across pages), preserving layer names and stroke-to-layer assignment.
• Export: each rnote DocumentLayer produces a named <layer> in the xopp output, with elements ordered to match rnote z-order (images below strokes). Empty named layers are written as placeholders; hidden layers are excluded.

UI (crates/rnote-ui)

A new "Layers" page in the sidebar with controls to:

• Add/delete layers
• Rename (inline entry)
• Toggle visibility and lock
• Move up/down
• Set as active layer

Layer buttons use symbolic SVG icons instead of text. Sidebar has i18n support for all layer-related labels.

Tests

Engine unit tests cover:

• Stroke insertion respects active layer
• Hidden layers are excluded from rendering/thumbnails
• Locked/hidden layers cannot be selected
• Layer order controls render order
• Layer mutations are undoable/redoable
• .rnote snapshot preserves layer metadata (name, order, visible, locked)
• Old snapshots without layer data assign default layer
• Renaming to same name is a no-op
• Xopp XML layer elements roundtrip with elements field
• Xopp import preserves named layers, merges across pages by name
• Xopp import handles unnamed layers correctly (index-based merge)
• Xopp export preserves layer names and excludes hidden/empty layers

Build verification

All targets pass:

• meson compile ui-cargo-check
• meson compile cli-cargo-check
• meson test (validation tests: 3/3 OK)
• cargo test --workspace (18 passed, 1 ignored)
• meson compile cargo-fmt-check (pass)