# Make imported/linked CSS always a full theme replacement

From WISHLIST.md: colour schemes should be full CSS replacements — there should be no "use as theme" tickbox distinguishing a selectable alternative theme from an always-on overlay. Every imported/linked stylesheet should just work the same way as the built-in colour schemes: a selectable, full replacement.

Currently `NavMenu.tsx`'s CSS section has an `is_theme`/"Use as theme" checkbox (`cssIsTheme`/`cssLinkIsTheme` state, `panel.importCssEntry(filename, content, isTheme)` / `panel.linkCssEntry(url, isTheme)`) that decides whether an entry becomes a selectable theme (validated against the canonical `--c-*` variable set, toggled via `panel.setCustomTheme`) or an always-on CSS overlay applied on top of the active colour scheme. This ticket removes that distinction: every imported/linked stylesheet is validated as a theme and becomes a selectable alternative to the built-in colour schemes, same as today's `is_theme: true` path.

## Acceptance criteria
- The "Use as theme" checkboxes are removed from both the CSS import form and the CSS link form in `NavMenu.tsx`.
- Imported/linked CSS is always validated against the canonical `--c-*` theme variable set (`parseThemeCss`) and always becomes a selectable theme entry (today's `is_theme: true` behavior) — the "always-on overlay" mode is removed.
- The explanatory text "imported/linked stylesheets — check 'Use as theme' to make one a selectable alternative to the built-in colour schemes above instead of an always-on overlay" is deleted/rewritten to reflect the simpler model (every entry is a selectable theme).
- The CSS filename input's placeholder changes from `"name.css"` to `"enter name"`.
- Backend/`CssEntry`/`is_theme` plumbing (`src/settings.rs`, `src/api.rs`, `src/dto.rs`, `webapp/src/lib/api.ts`) is simplified accordingly if `is_theme` becomes always-true and dead as a variable — remove it if no longer meaningful, or confirm it's still needed for backward compatibility with existing saved entries.
- Existing saved CSS entries (including any persisted with `is_theme: false`) keep working — decide and document how they're treated (e.g. migrated to always selectable) rather than silently breaking.