Add 24 syntax highlighting themes and pretty heading markers

[lmist]

Summary

• Adds a theme picker in the statusbar with 24 CodeMirror themes (Dracula, Monokai, Nord, GitHub, Solarized, Gruvbox, Material, Sublime, Andromeda, Aura, Copilot, VS Code, Xcode, Quiet Light, Tokyo Night variants) grouped by dark/light. Each theme includes matched app chrome colors and markmap branch palettes.
• Replaces raw #/##/### markdown heading markers with styled H1/H2/H3 badge pills that get progressively smaller per level. Badges hide when the cursor is on the line for editing.
• The sun/moon toggle button now acts as a quick dark/light swap, remembering your last choice in each category.

Test plan

• Cycle through all themes in the picker — editor, statusbar, and mindmap should all update
• Type headings at various levels and verify badges appear/disappear on cursor entry
• Toggle dark/light with the button and verify it swaps correctly
• Reload and verify theme persistence

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