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/MilestoneSelector.vue

@@ -51,7 +51,7 @@ function onChange(e: Event) {
 
 <template>
   <select
-    class="milestone-select"
+    class="fs-input milestone-select"
     :value="modelValue ?? ''"
     :disabled="!projectId || loading"
     @change="onChange"
@@ -64,23 +64,10 @@ function onChange(e: Event) {
 </template>
 
 <style scoped>
+/* The input itself is the .fs-input canon (components.css); only the
+   layout remainder lives here. */
 .milestone-select {
-  padding: 0.4rem 0.6rem;
-  border: 1px solid var(--fs-border-color);
-  border-radius: var(--fs-radius-sm);
-  background: var(--fs-surface-page);
-  color: var(--fs-text-primary);
-  font-size: 0.875rem;
-  font-family: inherit;
   box-sizing: border-box;
   width: 100%;
 }
-.milestone-select:focus {
-  outline: none;
-  border-color: var(--fs-accent);
-}
-.milestone-select:disabled {
-  opacity: 0.5;
-  cursor: default;
-}
 </style>