Docs: rewrite dashed clauses in the user guide as plain sentences #74

The README, CONTRIBUTING.md, SECURITY.md and the issue/PR templates were rewritten to drop em and en dashes where a plain sentence does the job. The user guide in apps/docs/guide still has 69 of them across 14 pages and should get the same pass.

Worst first: list-kinds.md (15), adding-todos.md (9), todos.md (8), colours-and-ordering.md (7), appearance.md (5), then lists.md, list-filter.md and install-on-your-phone.md (4 each), keyboard-shortcuts.md, index.md and getting-started.md (3 each), search.md (2), sound.md and offline.md (1 each).

Rewrite, don't substitute. The point is not to swap one mark for another. Most of these are a sentence wearing a disguise:

No account with me — you sign in with your existing CalDAV credentials.

becomes

There's no account with me. You sign in with your existing CalDAV credentials.

A colon is fine where the second half genuinely elaborates the first ("weighed against one question: do I want it?"). A comma works where the clause is a real aside. Full stops handle the rest, and usually read better.

Two things to watch, both of which bit during the README pass:

• Check the result reads as a sentence. One replacement left SECURITY.md reading "Use GitHub's [private vulnerability reporting] It opens a channel…", which parses as nothing at all.
• Run bun run docs:build afterwards. The dead-link check catches a link broken while editing prose around it.

Not urgent, and it does not block the public release.