refactor(frontend): the last pay-down, part 1 — three dead views deleted, editor rules shared, .page-container + .fs-input canon, rules-shared.css; a one-declaration CSS body is not a shape (#2903, milestone 299 step 5)

The derive queue said the biggest duplicate families were whole views:
TaskViewerView, UserManagementView and LogsView were imported nowhere — left
behind when tasks moved to the editor and users/logs became SettingsView
tabs. Deleted (rule 22). Note/TaskEditorView carried six identical scoped
rules -> editor-shared.css (the .tag-suggest-row gap the scoped copies
actually rendered wins). Three views wrapped the page under three names ->
.page-container in components.css. Three scoped input recipes -> the design
system fs-input recipe (snippet #2336), verbatim, in components.css; width/
box-sizing stay with the caller. The three rules panes share .pane and the
pane heading via rules-shared.css (the auth-shared pattern, #2852).

Ledger: a single-declaration CSS rule keeps its selector in its fingerprint,
so `color: var(--fs-text-tertiary)` under five different names is no longer
a five-file "identical body" family — the first pay-down found that most of
the 148 dup families were exactly this, and nobody would consolidate them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/rules/RulebookDetailPane.vue

@@ -121,10 +121,9 @@ watch(() => props.rulebookId, () => {/* re-render of isSubscribed from existing
   </section>
 </template>
 
+<style src="@/assets/rules-shared.css" />
 <style scoped>
-.pane { background: var(--fs-surface-hover); padding: 1rem; overflow-y: auto; }
 header { display: flex; align-items: center; justify-content: space-between; gap: 1rem; }
-header h2 { font-family: Fraunces, serif; font-style: italic; margin: 0 0 0.5rem 0; }
 .always-on-toggle {
   display: flex; align-items: center; gap: 0.4rem;
   font-size: 0.85rem; opacity: 0.85; cursor: pointer;