feat(editor): ${filename} in the image directory, so a folder can belong to one document (#714)

[PathGao]

Second half of #714. The session-persistence half is a separate conversation and is not touched here.

imageDirectory is read literally, so every document in a folder shares one img/. A directory of notes accumulates one flat pile of screenshots with nothing saying which document any of them belongs to, and renaming or moving a document takes none of its images with it.

What changed

${filename} in that setting now expands to the document's name without its extension. ${filename}.assets next to notes/trip.md writes to notes/trip.assets; each document gets its own folder. A setting without the token is still the literal folder name it has always been, and an empty setting still means img, so nothing changes for anyone who does not type the token.

Three call sites read the setting — paste (save_image), drop (copy_file_to_img), and the path completion that lists the image folder for ![](… suggestions. All three go through one resolveImageDirectory now.

Choices

The token is spelled ${filename} because Typora and SoloMD spell it that way. This request comes from people migrating off those, and the string they already have in their config is ${filename}.assets. Inventing a different spelling would mean every one of them has to be told about it.

Rust is unchanged, and that is the safety argument, not a convenience. The expansion produces a path component, not a path: trip.assets still goes through safe_path_component, which refuses separators, ., .. and absolute paths, and resolve_image_directory still asserts the result stays inside the document's directory. That guard is also why this function special-cases no stem of its own — a document named ..md expands to . and Rust is the one that refuses it, rather than a second validator in TypeScript that could disagree with the first.

Flat templates only. ./images/${filename}/ would require relaxing that constraint to allow separators, which is a different change with a different review. ${filename}.assets — the form the issue asks for, and Typora's own default — needs none of it.

Substitution is split/join, not String.replace. The replacement is a filename the user chose, and replace reads $& and $' in a replacement string as backreferences: a document named $&.md would have created a folder called ${filename}.assets on disk, containing the literal token. Pinned by a test.

One function rather than three inline expansions. The completion site is the one that makes this more than style: expanding at paste and drop but not there would list a directory named after the literal token, which exists nowhere, so image-path suggestions would silently return nothing for exactly the users who enabled the feature. A source assertion now fails if a call site resolves settings.imageDirectory by hand again.

The hint is English-only. The other 25 locales fall back to English, which i18nCoverage.test.ts documents as the supported path — the alternative is that no English string can be added without 25 translations first. It rides on the input's title, the tooltip pattern FindBar.svelte already uses; Settings.svelte has no description-text pattern, and inventing the first one there is a UI decision for its own PR. The placeholder stays img on purpose: it describes what an empty setting does, and ${filename}.assets in that slot would say otherwise.

Not in this change

• Rename. A renamed document leaves its assets folder behind under the old name. Typora moves the folder and rewrites the links; that is the larger half of per-document folders and needs its own design (what happens on a collision, on a partial move, on a folder the user has since edited).
• Failure reporting. A rejected or failed image write still only reaches console.error, as it did before for a full disk or a read-only directory. Unchanged behavior, worth fixing separately.
• Existing images. Nothing migrates; the setting applies to images written after it changes.

Tests

982 node --test green, svelte-check clean. New: two behavior tests over the expansion (Windows paths, the last-dot rule, .gitignore-style names, no-token and empty settings, and the $& case) and one source assertion that no call site resolves the setting inline.