feat(opensidian): toolbar follow-ups — onboarding, factory fix, logo

[braden-w]

Follow-up to #1632. That PR deleted the toolbar and redistributed actions. This one fixes what we got wrong and adds first-visit onboarding for the HN launch.

The terminal is the feature that makes opensidian different from every other notes app—but it was completely hidden behind ⌘`. A HN visitor would land on an empty file tree, see "No file selected", and leave. Now, first-time visitors get three things simultaneously: sample data populates the file tree, the README opens in the editor, and the terminal opens with a welcome message:

Welcome to OpenSidian — notes on CRDTs with a bash terminal.

Try these:
  echo "# Hello HN" > /hello.md    create a file
  ls /                              list files
  open /hello.md                    open in editor
  cat /hello.md                     print contents

80+ commands: awk, sed, grep, jq, find, sqlite3, curl, and more.
Press ⌘\` to toggle this terminal.

No modal, no tour, no signup wall. Just a terminal with a blinking cursor that a developer immediately understands.

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The sample data loader was hitting Svelte's `state_invalid_export` because it exported a bare `$state` that gets reassigned. The first workaround (a standalone `isSeeding()` function) broke the codebase convention where every `.svelte.ts` state module uses a factory. Reverted to a proper `createSampleDataLoader()` factory with the `$state` inside a closure exposed through a getter.

The "FILES" label in the sidebar header was dead weight—icons already communicate context. Removed. The Epicenter logo now sits at the left of the sidebar header in a `bg-black` rounded container so the gray+white circles stay visible on any background.

Stacks on #1632.