feat(palette): add Obsidian and Typora as consumers

Both expose a first-class, update-safe theming hook, which is what separates
them from the Electron apps that cannot be consumers: Signal, Discord and
Spotify ship closed bundles whose only theming route is patching a signed
asar that every update reverts.

Obsidian gets a CSS snippet rather than a theme, so it layers over whatever
theme a vault already uses instead of replacing the user's choice. Enabling it
means appending to enabledCssSnippets in each vault's appearance.json, which
bin/udt-appthemes does by reading the list and adding to it: rewriting the file
would have switched off the layout snippet one vault was already using.

Typora's theme is derived from the Dracula theme installed here, whose 350
rules all resolve through :root variables, so only that block is templated and
none of the layout is touched.

Verified across six vaults, nested ones included: the vault with an existing
snippet kept it, a vault with no appearance.json got a valid one, and three
consecutive installs leave exactly one udt entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015gbjA2bmswN8jyPDKzrvqe

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