Support local custom fonts

[i-am-the-slime]

Summary

• Adds an Upload local font button to the font picker so users can bring their own .woff2 / .woff / .ttf / .otf files (e.g. the TX-02 from Berkeley Graphics) when none of the bundled fonts fit.
• Uploaded fonts persist across reloads in localStorage (base64-encoded bytes) and appear in a new Custom group at the top of the picker with a per-row delete button.
• Custom fonts flow through both render paths: registered with document.fonts for the live DOM preview, and surfaced via loadCodeFont so Satori uses the same bytes when exporting PNG/SVG.

Implementation notes

• New src/lib/customFonts.ts holds the small store (one localStorage key, simple subscribe/notify), file-reader helpers, and a CSS-family helper.
• src/lib/fonts.ts now treats the font name as a string and falls back to the custom store when the name isn't in the built-in REGISTRY. Its load-promise cache is invalidated whenever the custom-font store changes so re-uploads pick up the new bytes.
• Settings.codeFont is widened from the CodeFont literal union to string. The union is still used as the key type of the built-in registry, so built-in fonts remain exhaustive.
• FontCombobox grows a file <input>, a custom-fonts group, and a remove button.

Test plan

• bunx tsc --noEmit clean
• bun run build succeeds
• Local dev: upload UI renders under the existing combobox; a persisted custom font shows up under Custom with a delete button, separated from Built-in below; selecting it updates Settings.codeFont.
• Upload an actual .woff2 (e.g. TX-02) and confirm both the live preview and the exported PNG render it.

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