docs: a guide, a wiki and screenshots

The README grew a section per feature and is a good reference, but nothing
said where to begin. docs/wiki now has a getting-started page, a guide that
walks through one day with the workspace and explains each part where it comes
up, and a page per part of the app, each with screenshots of what it
describes.

Three pages are read out of the code rather than written, so they cannot drift
from it: the keys and commands from Help.svelte and commands.ts, the same lists
`:help` draws; the settings from .env.example, which gains a heading for the
vault, trash and history settings so they no longer sit under mail; and the
API from the README's table. `make wiki` writes them, and `--check` fails when
one is stale or a link between pages leads nowhere. `--publish` lays the pages
out for a GitHub wiki: links without `.md`, images beside them, `[[` escaped so
the wiki does not take a command's help text for a link to a page.

The screenshots are taken by `make screenshots`: the debug binary on a
temporary data directory, with a password made up for the run, filled with an
example workspace over the API, and a headless Chromium driven over the
DevTools protocol through the views the docs show. Nothing outside the
temporary directory is touched but docs/images, and the pictures are cut to
256 colours, a third of their size, which a dark UI does not show.

The README opens with the banner, a CI badge, the guide, the features grouped
by what they are for, and six of the screenshots.

docs/wiki/Writing.md

@@ -0,0 +1,104 @@
+# Writing
+
+## The editor
+
+`i` or `a` on an open document edits it. The editor is CodeMirror 6 with the
+vim keymap: motions, operators, text objects, visual mode, registers, macros
+and `.` all work, and `:w`, `:wq` and `:q!` do what they do in vim.
+
+![The editor](../images/editor.png)
+
+- `:name` on its own moves up into the title; `Enter`, `Esc` or `↓` goes back
+  down into the text. `:name <text>` renames without leaving the text.
+- `]]`, `[[` and `gO` walk the headings, the same as in the reader.
+- `gf` follows the link under the cursor, saving first, and `gx` opens it in
+  a new browser tab.
+- `Ctrl-S` saves from anywhere, and `?` shows the help.
+- Yanks and deletes go to the system clipboard too. `:set noclipboard` keeps
+  them in vim's registers only.
+- `:set novim` turns the vim keys off for a plain editor; `:set vim` turns them
+  back on. On a touch screen they start off.
+
+Drag, drop or paste an image, a video or a sound file into the text and it is
+uploaded and embedded where the cursor is. `:upload` picks files to embed, and
+`:media` lists every upload: `Enter` embeds the one under the cursor, `y`
+copies its address.
+
+## Saving
+
+There is nothing to remember. Each change goes to the browser's storage as it is
+typed, and to the server a couple of seconds after typing stops.
+
+- When the server cannot be reached, the draft stays in the browser and the
+  save is tried again, less often the longer it fails, and at once when the
+  browser says it is back online.
+- When a session runs out in the middle of a sentence, the login appears, and
+  logging in again carries on where it stopped, draft and all.
+- When a tab dies in the middle of a document, opening the document again
+  restores the draft and says so. `:e!` throws the draft away.
+- When the document was saved somewhere else since you opened it (another tab,
+  the phone, the SFTP mount), `:w` says so instead of overwriting it. `:e!`
+  takes the other version, and `:w!` writes yours anyway.
+
+Each save keeps the text it replaces, so nothing written is lost. See
+[Not losing things](Not-Losing-Things.md#history).
+
+## Markdown
+
+Documents are GitHub-flavoured markdown: headings, lists, task lists, tables,
+quotes, code blocks and links. A single line break is kept as a line break.
+
+Math is set by KaTeX:
+
+- `$x^2$` inline;
+- `$$ … $$` on lines of its own, or a ` ```math ` block, for a display formula.
+
+![Math in a document](../images/math.png)
+
+## Links
+
+A document links to another in two ways:
+
+- by its address: `[the plan](/n/work/notes/plan)`;
+- by its title: `[[Plan]]`, or `[[Plan|the plan]]` with a label of its own.
+
+A title is looked for in the space the link is written in first, then in every
+other. It matches a name in any case, or the slug of that name. Typing `[[` in
+the editor offers the titles of documents as the search finds them. A title
+that no document has yet is drawn dimmed, and following it offers to write that
+document.
+
+Links keep working when what they point at moves. Renaming a document, or a
+folder above it, or moving either, rewrites the links to it in every document
+that has one, in the same save: the path to the new one, a `[[title]]` to the
+new name, the label left as it was. A link to something in the trash follows it
+when it is restored; once it is deleted for good the link text stays and leads
+nowhere.
+
+`gb` or `:backlinks` lists the documents that link to the open one, each with
+the line it does so on.
+
+A link written inside a card, or a bare address in prose rather than a markdown
+link, is not counted or rewritten.
+
+## Templates
+
+Any space can have a folder called `templates` at its root. Every document in
+it is a template.
+
+- `:new meeting` starts a document from `templates/meeting`. `Tab` completes the
+  template names.
+- `:new meeting 2026-10-20 Budget` dates it and names it as well.
+
+In a template these placeholders are filled in, on the clock of
+`WORKSPACE_TIMEZONE` at the date the document is for:
+
+| placeholder | becomes |
+| --- | --- |
+| `{{date}}` | `2026-10-20` |
+| `{{weekday}}` | `Tuesday` |
+| `{{time}}` | `14:05` |
+| `{{title}}` | the new document's name |
+
+A new diary entry starts from the diary's `templates/day` when there is one.
+See [The diary](Diary.md).