[Themes] Add identity default palettes for the image themes

The Fabric, Leather, Plasma and VelvetMarble themes shipped only zone art and no palette, so their chrome fell back to the bare OS palette. Each theme now ships light and dark curated defaults written in the palette editor's own conventions, with [AppColors] so the home-tab buttons keep the theme's identity over static backgrounds.

- Add palette-default-light.toml / palette-default-dark.toml for the four themes; scheme resolution follows the OS since the themes declare no color scheme.
- Tint the window/base/button chrome and bevels towards each theme's identity: navy cloth (Fabric), black-brown with brass accents (Leather), electric violet with cyan sparks (Plasma), charcoal velvet with slate marble (VelvetMarble).
- Fill [AppColors] AccentStrong / AccentSoft per theme so the home-tab button gradient matches instead of falling back to the default greens.
- Derive Disabled and Inactive groups with the same conventions as PaletteGenerator::fromAccent.

cockatrice/themes/Leather/palette-default-dark.toml

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+# Leather — dark identity palette (black-brown chrome with brass accents)
+[Palette]
+WindowText           = #eee9e2
+Button               = #332d26
+Light                = #4a4239
+Midlight             = #3d3730
+Dark                 = #131009
+Mid                  = #26221c
+Text                 = #eee9e2
+BrightText           = #e3c27e
+ButtonText           = #f5f1ea
+Base                 = #26221d
+Window               = #1d1a16
+Shadow               = #0d0b07
+Highlight            = #4a5f8f
+HighlightedText      = #ffffff
+Link                 = #cfa263
+LinkVisited          = #9a7db8
+AlternateBase        = #211d19
+ToolTipBase          = #37302a
+ToolTipText          = #f5f1ea
+PlaceholderText      = #6eeee9e2
+Accent               = #c9995a
+
+[Palette.Disabled]
+WindowText           = #9d9d9d
+Button               = #1d1a16
+Light                = #4a4239
+Midlight             = #3d3730
+Dark                 = #131009
+Mid                  = #26221c
+Text                 = #9d9d9d
+BrightText           = #e3c27e
+ButtonText           = #787878
+Base                 = #1d1a16
+Window               = #1d1a16
+Shadow               = #0d0b07
+Highlight            = #2d2822
+HighlightedText      = #9d9d9d
+Link                 = #308cc6
+LinkVisited          = #b450ff
+AlternateBase        = #211d19
+ToolTipBase          = #37302a
+ToolTipText          = #f5f1ea
+PlaceholderText      = #6eeee9e2
+Accent               = #9d9d9d
+
+[Palette.Inactive]
+WindowText           = #eee9e2
+Button               = #332d26
+Light                = #4a4239
+Midlight             = #3d3730
+Dark                 = #131009
+Mid                  = #26221c
+Text                 = #eee9e2
+BrightText           = #e3c27e
+ButtonText           = #f5f1ea
+Base                 = #26221d
+Window               = #1d1a16
+Shadow               = #0d0b07
+Highlight            = #2d2822
+HighlightedText      = #ffffff
+Link                 = #cfa263
+LinkVisited          = #9a7db8
+AlternateBase        = #211d19
+ToolTipBase          = #37302a
+ToolTipText          = #f5f1ea
+PlaceholderText      = #6eeee9e2
+Accent               = #4a5f8f
+
+[AppColors]
+AccentStrong         = #b7823b
+AccentSoft           = #e4c58f
+

cockatrice/themes/Leather/palette-default-light.toml

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+# Leather — light identity palette (black-brown chrome with brass accents)
+[Palette]
+WindowText           = #2a2114
+Button               = #dfcbb0
+Light                = #ffffff
+Midlight             = #c6b08c
+Dark                 = #a18a64
+Mid                  = #b29a74
+Text                 = #2a2114
+BrightText           = #7a531c
+ButtonText           = #1f1710
+Base                 = #fdf8ee
+Window               = #f0e2cb
+Shadow               = #5d4d33
+Highlight            = #34508c
+HighlightedText      = #ffffff
+Link                 = #8a5e1e
+LinkVisited          = #6b5190
+AlternateBase        = #f5ebd7
+ToolTipBase          = #fff7e8
+ToolTipText          = #1f1710
+PlaceholderText      = #7a2a2114
+Accent               = #a5712f
+
+[Palette.Disabled]
+WindowText           = #787878
+Button               = #f0e2cb
+Light                = #ffffff
+Midlight             = #c6b08c
+Dark                 = #a18a64
+Mid                  = #b29a74
+Text                 = #787878
+BrightText           = #7a531c
+ButtonText           = #969696
+Base                 = #f0e2cb
+Window               = #f0e2cb
+Shadow               = #5d4d33
+Highlight            = #ecd9bb
+HighlightedText      = #787878
+Link                 = #0000ff
+LinkVisited          = #ff00ff
+AlternateBase        = #f5ebd7
+ToolTipBase          = #fff7e8
+ToolTipText          = #1f1710
+PlaceholderText      = #7a2a2114
+Accent               = #787878
+
+[Palette.Inactive]
+WindowText           = #2a2114
+Button               = #dfcbb0
+Light                = #ffffff
+Midlight             = #c6b08c
+Dark                 = #a18a64
+Mid                  = #b29a74
+Text                 = #2a2114
+BrightText           = #7a531c
+ButtonText           = #1f1710
+Base                 = #fdf8ee
+Window               = #f0e2cb
+Shadow               = #5d4d33
+Highlight            = #ecd9bb
+HighlightedText      = #000000
+Link                 = #8a5e1e
+LinkVisited          = #6b5190
+AlternateBase        = #f5ebd7
+ToolTipBase          = #fff7e8
+ToolTipText          = #1f1710
+PlaceholderText      = #7a2a2114
+Accent               = #34508c
+
+[AppColors]
+AccentStrong         = #b7823b
+AccentSoft           = #e4c58f
+