viewport: local_viewport() opt-out of root-viewport inheritance; list_item: height/font_size builders

`Viewport::local_viewport()` resolves the child's viewport-relative (vw / vh / vmin) and fluid Lengths against the viewport's own rect instead of the root layout viewport the sub-canvas inherits since the fluid-resolution inheritance change. That inheritance is right for scroll-like clips (content renders the same size inside and outside), but it is exactly wrong for a fixed-size floating mini-UI — crustace's phone-shaped quick-settings pill pinned to a corner of a desktop-wide surface resolved its vw text and fluid stock geometry against the whole monitor, inflating the content past the pill's fixed clip and cutting off the bottom stripe. The flag pins the sub-canvas's layout viewport to its own size via the new crate-internal Canvas::set_local_layout_viewport, nested sub-canvases keep propagating the pinned value, and a unit test covers the override plus propagation.
`ListItem::height( impl Into<Length> )` and `ListItem::font_size( impl Into<Length> )` mirror the Toggle / Radio height() builders: override the theme row height (floored at the label's rendered height so the text never clips) and the primary-label font size (subtitle and trailing keep their theme sizes), letting dense context menus trade the stock touch-target generosity for row density.
Docs: widgets.md gains both builder sets, architecture.md documents the layout-viewport inheritance model next to the per-canvas density it parallels, the cookbook slide-in panel recipe notes when the pill needs local_viewport(), and the toggle / radio height() rustdoc demotes its private theme::HEIGHT link to a code span so cargo doc is warning-free again. CHANGELOG entries added.

CHANGELOG.md

@@ -6,6 +6,9 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 ### Added
 
+- **`Viewport::local_viewport()`** — resolve the child's viewport-relative (`vw` / `vh` / `vmin`) and fluid `Length`s against the viewport's own rect instead of the root layout viewport the sub-canvas inherits. For fixed-size floating mini-UIs (a phone-shaped panel pinned to a corner of a desktop-wide surface) whose content is calibrated against the panel rect; scroll-like clips should keep the default inheritance.
+- **`ListItem::height( impl Into<Length> )` / `ListItem::font_size( impl Into<Length> )`** — override the theme row height (floored at the label's rendered height so text never clips) and the primary-label font size, mirroring the `Toggle` / `Radio` `height()` builders, so dense menus can trade the touch-target generosity for row density.
+
 - **Per-canvas pixel density** — `Canvas::set_density` pins a canvas (and the sub-canvases derived from it) to its own density factor for `Length::dp` resolution, overriding the process `set_density` global; `Canvas::density` reads the effective value. New `Canvas::resolve_geom` / `Canvas::resolve_font` resolve an explicit `Length` in geometry / font space with the canvas' density — widgets now route caller-supplied lengths through them, so a `dp` override follows the canvas it draws on. The hook for surfaces on outputs whose DPI differs from the process-wide one (an overlay on a second monitor, an embedder with several `UiSurface`s).
 - **`Length::resolve_with_density`** — `Length::resolve` with an explicit density for `LengthBase::Dp`, instead of the process `density()`.
 - **`docs/backends.md`** — the canonical software/GLES capability matrix: what renders identically, what degrades gracefully on software (gradients, shadows, backdrop blur, bottom fade) and what is GPU-only, replacing per-method rustdoc archaeology. Linked from README, onboarding and the architecture known-gaps list.