Fix all remaining dark-theme colour bugs across the templates

Follow-up to the player-modal / maintenance-page fix: the same root cause --
colours hard-coded for the light theme -- affected the whole app, in two
places.

1) Inline style attributes (inline outranks the theme rules, so the light
   value survives dark mode and the inherited text colour turns
   light-on-light). base.html now carries a single, property-scoped
   dark-mode override layer that maps every light literal found in the
   templates to the theme variables: light backgrounds/whites -> dark
   surfaces, semantic pastels -> dark equivalents, dark text -> light text,
   light borders -> dark borders, plus .back-link. Scoped to
   body.dark-mode, so the light theme is untouched. This covers all
   templates, including ones not edited here and any added later.

2) Page-level <style> blocks with no dark support:
   * admin/https_config.html hard-coded a light-only palette (blue-tint
     panels, dark labels) -- added a body.dark-mode block for it.
   * dashboard.html / help.html / manage_player.html coloured text links
     with --primary-color (#7c3aed in dark = too dim). Introduced a
     dedicated --link-color (#667eea light / #a78bfa dark) and used it for
     text links, leaving --primary-color for button/background use.

Verified with an automated luminance scan of all 18 pages in both themes:
dark mode has 0 light backgrounds and 0 dark-on-dark text (the only
remaining light surfaces are the intentional white buttons on the admin
gradient cards), and light mode is unchanged.

app/templates/dashboard.html

@@ -315,7 +315,7 @@ body.dark-mode .compact-btn:not(.btn-success) .compact-btn-icon {
 .playlist-name {
     font-weight: 600;
     font-size: 1rem;
-    color: var(--primary-color);
+    color: var(--link-color);
     text-decoration: none;
     word-break: break-word;
 }
@@ -374,7 +374,7 @@ body.dark-mode .player-chip.is-offline .status-dot { background: #718096; }
     font-size: 0.85rem;
 }
 .unassigned-note a {
-    color: var(--primary-color);
+    color: var(--link-color);
 }
 
 /* Sidebar collapses below the main column when space runs out. */