Palette Editor example (#6490)

* Add initial palette editor app layout

* Add themed control showcase to palette editor

* Add interactive color swatches to palette editor

* Add color editor pane to palette editor

* Add material color picker pane to palette editor

* Add 'Reset from seed' button (stub)

Insert a no-op reset_from_seed handler and add an IconButton (Icons.PALETTE) with tooltip 'Reset from seed' to the palette editor UI. The handler is currently a placeholder (pass) and should be implemented to restore palettes from a seed. Change made in sdk/python/examples/apps/palette_editor/main.py.

* Reset palette editor theme from seed

* Add shade picker to palette editor

* Refine palette editor shade strip

* Copy palette theme export to clipboard

* Show palette export dialog

* Highlight selected palette roles

* Sync palette editor role selection

* Use hue ring picker in palette editor

* Align palette editor pane to top

* Improve palette export interactions

* Expand palette editor theme preview

* Show secondary palette roles in preview

* Add tertiary palette roles

* Organize palette preview into role tabs

* Show default palette colors in editor

* Refactor palette editor layout builders

* Add palette editor theme mode toggle

* Separate palette overrides by theme mode

* Confirm palette seed rebuilds

* Add error palette roles and previews

* Reorganize palette editor role groups

* Scope palette theme to preview pane

* Update palette swatches for active theme

* Refine palette editor preview controls

* Refine palette editor swatch layout

* Adjust palette editor label theming

* Keep palette group chrome on system theme

* Clarify palette import and export theme mode

* Add scaffold background palette preview

* Document surface palette role hints

* Refine page background theme support

* Refine scaffold background editor copy

* Add hint for Page background in palette constants

Add a "hint" entry to the "Page background" tab in sdk/python/examples/apps/palette_editor/palette_constants.py. The hint provides the tooltip text "Customizes the page background color." to clarify the control's purpose in the left pane role tabs.

* Expand palette editor role descriptions

* Refine palette editor role grouping

* Autofocus palette import dialog

* Centralize palette role descriptions

* Move palette theme toggle to left pane

* Refine palette editor panel surfaces

* Reorganize palette preview tabs

* Refine palette surface previews

* Relax palette import parsing

* Refine palette editor metadata

* Rework palette all-tab preview shell

* Rebalance palette surface add-on previews

* Refine palette all-tab mockup layout

* Refine palette preview accent cards

* Reclassify palette_editor example to Tools

Update [tool.flet.gallery] categories in sdk/python/examples/apps/palette_editor/pyproject.toml: change category from "Apps/Basic" to "Tools" so the Palette Editor example is listed under the Tools section in the Flet gallery.

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Co-authored-by: Feodor Fitsner <feodor@appveyor.com>

Author: InesaFitsner