Feature/unified canvas - #198

Mirrors the hasSeenSliderHint pattern in OnboardingManager — StateFlow +
markCanvasOnboardingComplete() backed by a new "canvas_onboarding_seen"
key in onboarding_prefs. First of four steps for the Canvas tab's
first-open guided tour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Two-step guided tour copy: step 1 introduces Canvas and points at the
menu button; step 2 explains the long-press-to-open-options gesture and
overviews what's inside. Also adds a shared onboarding_skip label since
the CanvasOnboardingOverlay exposes a Skip control the header overlay
does not.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Mirrors HeaderOnboardingOverlay — opaque black scrim, target redrawn
above via LayoutCoordinates offset+size, OnboardingTooltip centered.
Dispatches on targetId (canvas_menu_button / canvas_area) to pick the
right coordinate slot and content lambda.

Adds a Skip control in the top-end corner (calls onComplete) since the
brief requires one for the Canvas flow — kept local to this overlay
rather than modifying the shared OnboardingTooltip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Plumbs an optional onAddTilePositioned callback from CanvasGrid down to
AddTile, which reports its LayoutCoordinates via onGloballyPositioned.
UnifiedCanvasTab wraps CanvasGrid's modifier with onGloballyPositioned
to capture the canvas-area coordinates in parallel.

The overlay shows when the Canvas tab becomes the active pager page,
CanvasUiState is bound (pageIndex != -1), resolvedItems is empty, and
hasSeenCanvasOnboarding is false. Advancing step 1 also closes the add
dialog so the empty canvas is visible for step 2 as the brief requires.

onAddClick is intercepted: during step 1 it no-ops (focus stays on the
menu button); during step 2 the long-press auto-completes onboarding
and then opens CanvasMenuDialog as the natural payoff. The tooltip's
Done button on step 2 completes without opening the dialog. Skip in the
overlay corner also completes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>