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>

tests/test_pattern_coverage.py

@@ -484,12 +484,18 @@ def test_extract_definitions_fingerprints_each_block():
     d = {x.name: x for x in extract_definitions(css)}
     assert d["closed-msg"].body_sha == d["error-block"].body_sha != d["other"].body_sha
     # One-line rules hash their own declarations — never the empty string
-    # (first deploy grouped 68 unrelated one-liners as one copy).
-    one = ".a { color: red; }\n\n.b { color: red; }\n\n.c { color: blue; }\n\n.d {\n  color: red;\n}\n"
+    # (first deploy grouped 68 unrelated one-liners as one copy) — and a
+    # SINGLE declaration is not a shape (#2903): it keeps its selector in the
+    # hash, so `.a { color: red }` groups only with another `.a`, never with
+    # `.b { color: red }`. Two declarations and up stay selector-agnostic.
+    one = ".a { color: red; }\n\n.b { color: red; }\n\n.c { color: blue; }\n\n.a {\n  color: red;\n}\n"
     e = {x.name: x for x in extract_definitions(one)}
     import hashlib
-    assert e["a"].body_sha == e["b"].body_sha != e["c"].body_sha
+    assert e["a"].body_sha != e["b"].body_sha != e["c"].body_sha
     assert e["a"].body_sha != hashlib.sha1(b"").hexdigest()[:16]
+    two = ".a { color: red; margin: 0; }\n.b {\n  color: red;\n  margin: 0;\n}\n"
+    f = {x.name: x for x in extract_definitions(two)}
+    assert f["a"].body_sha == f["b"].body_sha
 
 
 def test_coverage_line_names_the_proposers_standing():