Make imported/linked CSS always a full theme replacement (ticket 020)

Removes the "Use as theme" checkbox/is_theme distinction from
NavMenu.tsx's CSS import and link forms — every imported/linked
stylesheet is now always validated against the canonical --c-*
variable set and becomes a selectable theme, same as today's
is_theme:true path. Drops is_theme entirely from the backend
(CssEntry, ImportCssRequest, LinkCssRequest, CssEntryDto) and from
the frontend CssEntry type; an existing settings.json with a stored
is_theme:false entry still deserializes fine (the field is simply
unknown/ignored) and that entry becomes selectable like any other,
so no migration step is needed. Also drops the now-dead
always-on-overlay CSS injection path from useSettingsPanel.ts, and
changes the CSS filename placeholder from "name.css" to "enter name".

Implemented directly against main rather than merging the dispatched
worktree branch for this ticket, since that branch's base predated
ticket 012 (the custom-themes/imported-CSS consolidation) and its
NavMenu structure no longer matched current main.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/api.rs

@@ -392,13 +392,7 @@ async fn import_css(
 ) -> impl IntoResponse {
     let css_dir = state.css_dir();
     let mut settings = state.settings.lock().unwrap();
-    match settings.import_css(
-        &state.settings_path,
-        &css_dir,
-        body.filename,
-        &body.content,
-        body.is_theme,
-    ) {
+    match settings.import_css(&state.settings_path, &css_dir, body.filename, &body.content) {
         Ok(id) => {
             let entry = settings
                 .css_entries
@@ -417,7 +411,7 @@ async fn link_css(
     Json(body): Json<LinkCssRequest>,
 ) -> impl IntoResponse {
     let mut settings = state.settings.lock().unwrap();
-    match settings.link_css(&state.settings_path, body.path_or_url.clone(), body.is_theme) {
+    match settings.link_css(&state.settings_path, body.path_or_url.clone()) {
         Ok(()) => {
             let entry = settings
                 .css_entries