Themes as standalone config files + configurable shadow + dim unfocused + resize content-loss fix

- Theme is now a runtime struct loaded from its own TOML file
  (theme = "themes/dark.toml" in config.toml, like panels), all
  fields optional with built-in defaults; ships themes/dark.toml and
  themes/light.toml
- shadow = true/false in config.toml, shadow_color per theme
- dim_unfocused (per theme): unfocused window content is dimmed
- BUGFIX: resizing a window no longer erases its content and history —
  the vt100 parser was recreated on every resize; now uses
  screen_mut().set_size() which preserves the grid and scrollback
- New integration check: window content survives mouse resize (20/20)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

themes/dark.toml

@@ -0,0 +1,19 @@
+# TUI-WM standardtema (mörkt).
+# Välj tema i config.toml:  theme = "themes/dark.toml"
+# Alla fält är valfria — utelämnade fält får inbyggda standardvärden.
+# Färger: namn ("cyan"), index ("235") eller hex ("#1e1e2e").
+
+accent       = "cyan"    # fokusram, logga, markeringar
+resize       = "yellow"  # ram under pågående storleksändring
+resize_hover = "220"     # ram vid hover över kant
+border_dim   = "242"     # ofokuserad fönsterram
+desktop_bg   = "235"     # skrivbordsbakgrund
+panel_bg     = "238"     # panelens bakgrund
+menu_bg      = "236"     # dropdown-menyns bakgrund
+text         = "white"
+panel_text   = "252"
+hint         = "240"
+ok           = "green"
+danger       = "red"
+shadow_color = "233"     # fönsterskuggans färg
+dim_unfocused = true     # dämpa innehåll i ofokuserade fönster

themes/light.toml

@@ -0,0 +1,17 @@
+# TUI-WM ljust tema — exempel på eget tema.
+# Välj i config.toml:  theme = "themes/light.toml"
+
+accent       = "blue"
+resize       = "166"
+resize_hover = "208"
+border_dim   = "249"
+desktop_bg   = "253"
+panel_bg     = "251"
+menu_bg      = "254"
+text         = "black"
+panel_text   = "236"
+hint         = "245"
+ok           = "28"
+danger       = "124"
+shadow_color = "248"
+dim_unfocused = true