Rebuild /dashboard with charts, add Grand livre/Dashboard nav

Nav: a small "Grand livre" / "Dashboard" switcher, top-right on both
pages, with the current page highlighted. Needed hook_theme() to
declare current_route as an accepted variable for both theme hooks --
same class of bug as the earlier can_view_history fix: an undeclared
variable passed via the render array is silently dropped rather than
reaching Twig.

Dashboard: replaced the old "solde par compte / par client" tables
(computed client-side from a full unwindowed JSON:API fetch of every
node -- the same performance problem the /lignes sliding window was
built to avoid, just not yet felt at 1500+ lines) with a single
aggregate endpoint (DashboardStatsController, plain SQL GROUP BY) and
five chart panels: CA par année, solde par compte (diverging,
red/green), solde par compte trend (small multiples per compte),
répartition par type, top clients par CA. No charting library --
small dependency-free div/CSS bar charts (HBarChart/ColumnChart/
MiniTrend components in dashboard.js), consistent with this project
vendoring its own JS.

Two data-correctness fixes along the way: (1) the historical stray
mistyped dates (0213-06-15, 2015-08-29 -- preserved as-is per this
module's policy) needed excluding from per-year buckets without
excluding their money from all-time totals, so the filtering happens
per-output-field in PHP rather than as a blanket SQL date range. (2)
PHP silently casts numeric-looking array keys ("2021") to actual
integers, so array_keys() on a year-keyed map produces a mix of ints
and strings -- json_encode emits the int ones as bare JSON numbers in
a list (unlike object keys, which JSON always stringifies), which
broke the frontend's annees[i].slice(2) trend-card labels. Fixed with
an explicit array_map('strval', ...).

Verified: bar widths/colors match the underlying data exactly (e.g.
EXT.'s red bar is proportionally sized against Maud's green one per
their actual solde ratio), all 8 trend cards render correct year
labels, and both nav links correctly highlight on their own page.

web/modules/custom/figli_compta_ledger/css/admin-chrome.css

@@ -13,6 +13,45 @@
   margin-block-start: 0 !important;
 }
 
+/* Grand livre / Dashboard switcher, top-right of both pages. Only present
+   on those two custom routes, but attached here (already loaded on every
+   page for the phantom-top-bar fix above) rather than duplicated between
+   home.css and dashboard.css -- and defined with its own explicit colors
+   rather than relying on either page's #figli-*-app-scoped custom
+   properties, since this <nav> sits outside both of those elements. */
+.figli-page-nav {
+  display: flex;
+  justify-content: flex-end;
+  gap: 0.4rem;
+  padding: 0.5rem 0.25rem 0;
+}
+.figli-page-nav a {
+  padding: 0.3rem 0.8rem;
+  border-radius: 6px;
+  font-size: 0.82rem;
+  font-weight: 600;
+  text-decoration: none;
+  color: #6b7280;
+  border: 1px solid transparent;
+}
+.figli-page-nav a:hover {
+  border-color: #dcdee2;
+}
+.figli-page-nav a.is-active {
+  background: #eef1f6;
+  color: #1a1a1a;
+}
+html.gin--dark-mode .figli-page-nav a {
+  color: #a1a5ab;
+}
+html.gin--dark-mode .figli-page-nav a:hover {
+  border-color: #3d3e42;
+}
+html.gin--dark-mode .figli-page-nav a.is-active {
+  background: #2c2d30;
+  color: #e8e9ea;
+}
+
 /* Drupal status messages: fixed top-right, out of document flow, so they
    never shift the page layout the way the default in-flow placement does
    -- and shrunk to a fraction of Gin's default size, which is sized for a